Trading or selling personal camera equipment while temporarily in the US

The key link appears to be Transporting and importing commercial goods to sale in the U.S.

There are various sections from that link that seem to apply to your situation.

First of all, your camera equipment was being brought into the US for commercial use (my highlight)

As a general rule, yes; there is a significant difference between goods that a traveler brings into the United States (U.S.) for personal use and goods that someone brings in for commercial use, i.e. for sale.

It would seem that this was either communicated to the CBP in Bermuda or that they suspected it.

Next bit of information:

The value of your goods is also a critical factor. If the total value is over $2,500 - no matter what the items are - a formal entry will be required.

Given that you seem to be selling a significant quantity of camera equipment it would really be easy to exceed $2500 (otherwise why are you personally hand delivering it to the store?). I can look around my office here and I could easily point out $2500 of camera equipment that I have collected over the years - and I am an amateur.

However they do have this to say about lesser valued goods:

Items that are not subject to any special agency requirements might be eligible to be entered as an informal entry - as long as their total value is not over $2,500. In this case, after you have declared the items, a CBP officer will determine the amount of duty owed and refer you to the cashier to make payment by cash, certified check or - at the larger airports, credit card.

And that brings us to what is "a formal entry". I'm sure there is a definition of this term somewhere, but from the link I have been quoting they offer 3 scenarios to meet this requirement.

1. You can make arrangements with the airline to handle your goods as freight and to arrange Customs clearance on your behalf.

2. You can make your own arrangements ahead of time for a customs broker to meet you at the terminal with all the required paperwork ...

3. You can attempt to clear the goods yourself. This entails surrendering the goods to an airline representative - who will make arrangements to have them taken to a cargo facility under CBP supervision ...

Now you haven't detailed how you were attempting to bring your equipment into the US, but from the above, attempting to do so via carry on or checked luggage would not meet the above criteria and is probably why you were denied boarding.

Basically the US government wants a cut of your profits by applying the requisite import duty. By not going through official channels you are trying to deny them their cut. And they don't like that :D