He didn't steal anything from you.
You sold it to him, and you learned a valuable lesson.
Do your research BEFORE you sell something.
Not after.
Was he unethical? Yep. But that's a sad fact of life. The only person who is going to protect you from unethical people is... you.
That said, I do feel badly for you. I've been involved in the periphery of the antiques business for a long time, and it's common to see this kind of thing.
It's also common for someone to sell something and get a fair price for it (for example, $100), but later claim to have been robbed because the dealer didn't give them the retail price of $250.
As Rick on the show Pawn Stars says to many people who come into his shop, "I wouldn't stay in business if I gave everyone retail price. I have to buy it for a price that will let me make money on it."
Look at it this way. Along with your lesson, you've got $250 that you didn't have before. Given that you had no, or little, money in the sights in the first place, you have $250 of pure profit. Not much solace, but it's something.
I'm going to close thread for the simple reason that threads like these often turn into a feeding frenzy of people bashing the original poster.