Auction winner in one word

Solution 1:

The generally adopted term for this is successful bidder as recognized in commercial and, probably also, legal contexts.

Strictly speaking, the highest bidder is considered as the tentative 'winner'. If his bid is technically cleared and no objections are raised, he would be the 'successful bidder'. He may or may not eventually buy -- if he completes the formalities and pays up, he would be the 'buyer'.

A single word may be possible but can be a problem being different from the well-understood phrase.

Solution 2:

Buyer? Only the top bidder can actually buy the auction lot.

Winner? There is only one winner (and you actually used the word won yourself).