I've never bought a pin yet from a public forum like CL or Facebook where after I've got a tentative deal from a seller, some A-hole flipper in the pinball community calls the seller up, finds the item is "sold", and proceeds to tell the seller that the item is worth MUCH more than he listed it for and offers a hundred or two more than me. Then, more often than not, I'm in an auction.
A very very few sellers will honor their agreement with me regardless of subsequent offers. Most will generally beat me up for another couple hundred and then honor their agreement. A few will just open the auction for bidding, and let the highest bidder win! I usually walk away from those. No surprises here with your situation. Some people just can't stand seeing another person get a good deal instead of them, and are bound and determined to screw everyone else over until they get the deal. So be it.