Yes, if you sell something in an online auction, you create a contract.  When you put the item up for sale, you are making an offer to sell it for a certain price.  Typically, there is a minimum price that will be accepted.  The offer is made to anyone who sees your site.  Once someone agrees to pay that price, you have acceptance.  With an auction, there may be several bidders but only one successful buyer.  Still, once the auction closes, you have a contract for the sale of the item.  You have an exchange of promises to sell and to buy, and you have valuable consideration for each.