Is it possible to enter the US on a one-way ticket?

You are required to be a possession of a return or onward ticket (Canada, Mexico and the Caribbean don't count unless you're a resident), and airlines are likely to deny you boarding if you don't have one (which they can see electronically), since it's written in Timatic (the above link), the database used by Airlines which is based on info from immigration authorities worldwide

Visa required, except for Passengers with a biometric British passport with nationality of British Citizen shown on the bio-data page. They must have an Electronic System for Travel Authorization (ESTA) and travel as a tourist, on business or in transit, for a maximum stay of 90 days. (SEE NOTE 60190) NOTE 60190: Passengers must hold a return/onward ticket.

However, the CBP (US immigration) is unlikely to ask for it unless they decide to examine you more closely for some reason.

You could buy a refundable return or onward ticket (though not to Canada, Mexico or the Caribbean) to overcome this.

Again, though, it will probably mainly be the airline who asks for it. Having entered the US a number of times, I've been always been questioned a fair bit, and sometimes downright grilled, but never outright asked to show anything but my passport and APC receipt. Then again, it might be a different story if taken aside for secondary, which has never happened to me

Yes. I did (from Toronto to Chicago, as part of a wider journey stopping in NYC, Niagara Falls, Toronto, Chicago, Long Beach and San Francisco, all over about two and a half weeks). I even got a complete new 90 days at Toronto. Though this caused a bit of a grilling about my JFK entry stamp since the exit date was the wrong year (I actually asked the CBP guy at JFK and they said it didn't matter). I didn't however have to prove anything by showing tickets or anything like that (and certainly didn't have to pay bribes). I did have an onward ticket (San Francisco to Heathrow) but I can't see how Toronto would have known.]

Maybe this is different though because I was already in North America at the time? Also one-way transatlantic air tickets are a lot more expensive than return tickets.

I've seen it in a sense done--some of my wife's relatives were visiting (on a normal round trip ticket) and we took them up to Canada on a side trip. While that ticket was a round trip ticket the last leg was used to fly back to the US--while they still had valid tickets home the airline certainly didn't know about it and it's unlikely immigration did. Nobody asked.

Travel Agenies [at least in Germany] do not offer one-way tickets to USA for non-US citizens (not even to greencard holders). One-way ticket costs usually up to three times more than a roundtrip ticket. Nobody with common sense would pay this. Seems to serve the purpose that if an alien is found inadmissable into the U.S. upon arrival for whatever reason, he or she can instantly be put on the next airplane back to the country of origin. As I understand it, it's the airlines' policy to avoid a huge fine in case they boarded a passenger with inadequate travel documents, or records that might render him or her inadmissable.