First, if the auction is a Reserve Price Auction and the reserve is not met, the seller is not obligated to sell the item to you.
If this is a regular auction without a reserve price or seller exclusions (for example: seller states they won't sell to bidders with negative feedback), the seller is obligated to sell to you. However, eBay can't force someone to sell, so if your seller still refuses, you may leave this person appropriate feedback.
