I have been once to 1stDibs in NY and I have to admit, everything on display there was absolutely stunning but indeed, the prices are something else.
I had several conversations with dealers, not only there but other places in NY too and it was quite interesting. They all agreed that a lot of dealers focus on a specific public, often interior designers out on the hunt for a client. People with money who don't care how old something is and if it was made last week or a 100 years ago. If it has the look that they're after then they're happy to pay whatever the price is, no questions asked.
The Manhattan Art & Antiques Centre is very similar, very nice to visit and to have a look but you don't want to buy anything.