ban-all-sheds said:
Once the auction is over, he is obliged to sell it to you for the closing price. But there's SFA you can do to make him if he won't.
It's correct that you can't make him sell it, but that's because the agreement isn't a binding contract at the close of the auction.
This whole thing of an auction agreement being a legally binding contract is a popular myth. In English Contract Law, a contract is formed only when an offer is accepted. (There are other criteria too, but for clarity here I'll be brief).
There are cases where the offer comes from the seller, but in most cases, including auctions, the offer comes from the buyer.
Therefore, the contract is neither made, nor is its performance enforceable, until the seller acknowledges the closing price and agrees to accept it. This applies regardless of whether a reserve has been set.
So, you might well win the auction, and think that you've got the chalet "for a song", but the seller could then pull out simply by declining to accept the final price. He would face negative feedback, of course, and the wrath of the eBay organisers, but you cannot force him to accept your offer.
BTW, the eBay rule, the one preventing a seller from pulling out within the last 12 hours of auction end, is simply an eBay rule; it isn't anything to do with the law.