If the auction ends with a valid bid then legally it's a valid contract. If you cancel the sale then in theory you are in breach of contract and could be taken to court - realistically that is very unlikely, but it could happen. If you cancel the sale you get an account defect with eBay, do it too often and eBay will suspend your account.
If using auctions set the minimum bid to the lowest you would be willing to sell the item for. If the lowest value is the same amount that similar items sell for as Buy It Now, ask yourself whether you really want to auction it off. Auctions of normal items generally sell for less than other Buy It Now items - why would anybody pay more for yours if same is available BIN for less. To me auctions only make sense for rare items where price is hard to assess (rare artwork etc.). If you have an item you want to get rid of, you may be better doing a "reverse auction": list it as Buy It Now then gradually reduce the price until it sells e.g. drop price every few days.
