What happens if I bring 100+ of the same item in my luggage with the intention of selling?

If the perfumes are liquids, you may not be allowed to bring a large quantity of them in a carry on bag. A checked bag should be fine from a security standpoint; however as Nate Eldridge points out in the comments there may be a limit as to how much you can bring if they are considered flammable.

The bigger issue will be customs at the destination country. Generally if you are bringing more than a certain value of items, or if you are bringing items with intent to sell them, you will need to declare them to customs on arrival. Depending on the laws of the destination country, you will likely be required to pay an import tax or duty on the value of the items you bring in for sale. In some cases, you may also need a license to import goods for sale. Failure to follow the rules of the destination country can lead to heavy penalties or even prosecution. You should carefully research the rules of the destination country to ensure that you comply with their rules. In short, it's not something you should consider just doing on a whim.

This is not an issue for airport security (unless you violate the limits they define), they couldn't care less if you have 500 pairs of underwear or such.

It's an issue for Customs at arrivals. Most countries limit the things you can bring for free, and require you to pay import duty for articles you plan to sell. Bypassing this is called 'smuggling'.

It depends on your destination country what is limited and how.

They'll let you fly. And make you fly home!

Entering a foreign country is not a right, unless you are a citizen there.

They'll let you board the plane. But either at your destination or enroute, you will have to go through Immigration (which is looking at you) and Customs (which is looking at your stuff).

You will be asked the purpose of your trip, and whether you have anything to declare. If you disclose that you have all those perfumes and that you are there to sell them, then Immigration will refuse you entry into the country because you are entering on a visa that doesn't allow that kind of trading.

If you do not disclose both things, then Customs will pull you aside because of what they saw on the X-ray, and they will make you open up your bags et voilà. Now caught, you will be taken back to Immigration, who will refuse you for above reasons, and also deception. Deception is a show-stopper in immigration, it typically results in a very long ban.

You must now pay walk-up prices for a flight home.

You do not get to enter the country. You must now immediately book an immediate flight home. You will most likely be spending time until the flight in the immigration detention area.

The airline that flew you there is obliged to take you back, but can charge you any price they please. Typically that is "full-boat retail" - as you may know, most airline tickets are heavily discounted. It might be free if you have a full-service ticket that allows date changes and you are able to reach their ticket desk and change it. But discount travelers usually don't spend the money for those.

Regardless, most likely you will be spending the time from now until the return flight inside the immigration interview/detention area. And then be escorted to the gate at the relevant times.

If you are lucky, the state may allow to stay in the international departures area of the airport (including any hotel there) until flight time.

If you are very, very lucky, they will grant you entry solely to wait for the flight at a normal hotel "in town". This is a trap! It's awesome data for them: if you comply, they know you're trying. If you flake out, then forget ever getting a visa approved.

"But I don't need to apply for a visa! My country has a visa waiver agreement!" Heh heh.

More repercussions

If you came in on a visa waiver, you won't be doing that anymore because of the refusal, and must now apply for a visa for any future trips. Applying for visas is expensive. It will also prejudice your entry into other countries with which this country shares data.

The official way to do this, as other answers have pointed out or alluded to, is to check with customs (and probably immigration) of the country you intend to sell the items in beforehand what the legal terms are. I assume that practically all countries will legally require a declaration of the goods you are importing to customs and will require you to pay import taxes and/or customs duties. Many will require some kind of business id (e.g. you may have to have a company set up that is able to do international trading or simply to legally sell the goods after bringing them in) and/or an appropriate (business) visa if you are not a citizen of the country (assume that no visa waiver agreement will cover any of this). You can pretty much expect the fees, taxes and duties you have to pay to eat up practically all of your profit margin.

Anything outside of the aforementioned legal pathway is illegal and considered smuggling. It can get you into serious trouble, especially if you are trying to smuggle into a country that you are not a citizen of.

That said, your odds of getting caught will vary wildly based on such things as:

• the type of goods you are trying to smuggle
• the extent to which arriving passengers' luggage is screened
• the way immigration but especially customs inspections are set up (both in the destination country as a whole but also the specific airport)
• your luck on a specific day

Possibly one of the easiest ways to not get caught would be to fly into the EU as a citizen and have an intra-Schengen (but international) connecting flight as your final leg. That way, to local customs it is a plane arriving from an city where no inspections are typically necessary and no passenger is expected to have anything to declare. (While EU customs are generally very lenient towards air passengers, there are cases of planes coming in from common holiday destinations where travellers are more likely to pick up illegal goods or attempt to smuggle something in which are then completely screened on arrival; i.e. the Nothing to Declare Lane is blocked off. You definitely do not want to be on one of those.)

On the other hand, I remember arriving in one country and having to put my hand luggage down on the ground at some point between disembarking and leaving the airport to allow a customs dog to pass by everything; in such a jurisdiction, expect to be caught every single time especially if it is something the dog is expected to smell. I would especially expect countries like Australia and New Zealand -- which have very strict rules on what may and may not be brought in and do their best to enforce them -- to catch you and make you regret.

I will repeat that smuggling items past customs like this is illegal and will have severe repercussions if you are caught. I do not condone this activity.

A word on airport security since you mentioned hand luggage.

There are two instances where you may be looked at. Number one is at check in. Airlines only allow a certain amount of hand luggage and there is typically also a weight restriction. I have been told to weigh my hand luggage, it was found too heavy and parts of it had to be added as hold luggage; I had to pay the full fee for additional hold luggage that I required. Check in will also remind you of certain items that are prohibited in hand luggage (such as sharp objects, and liquids), certain items that are prohibited in hold luggage (powerbanks, most items with lithium ion batteries) and certain items that are prohibited on the plane together (explosives). They will typically not check your luggage on the spot though.

Number two is airport security. They, however, will only care for the security aspect of things. i.e. no knives, blades or similar sharp objects, no explosives, and no liquids outside of the permitted amount (your transparent bag containing flasks of a certain size and not exceeding a certain combined volume). Assuming you intend to take perfumes, security will find them and *will refuse to let you pass because perfumes are, obviously, liquids. They typically won't mind if you take, e.g. 50 copies of the same book with you because paper isn't considered a weapon of any sort. They also won't care about the weight of your luggage because different airlines have different limits and it's not their job to check.