They are under an obligation to sell you the item however if they choose not to because you won it for less than they were expecting eBay will only give them a "strike".
3 strikes and they are banned iirc.
I tend not to buy anything unless I have had contact with the seller beforehand.
There are a lot of idiots on eBay these days and all eBay seem to care about is getting the money rolling in, do they have any other items for Sale
What is their User identification?
There are ways of finding them, just don't leave bad feedback until you know the circumstances.
When buying I like to collect ASAP so I contact seller beforehand to let them know, sometimes this isn't a good move as it can show the seller your more interested in an item and it can induce shill bidding.
Ebays not really as good ad it used to be, too many assholes and scammers.