In a regular, lift-your-hand-to-scratch-your-nose-and-you've-placed-a-bid auction a seller can put a reserve or minimum amount on an item. Unlike eBay, this reserve is usually the opening bid, not a hidden amount that must be reached before a sale is consummated.
In a "real" auction, once an item goes under the hammer, that is, once the bidding has been started, it is not possible to withdraw it from the auction. If the seller withdraws it before it comes up for bidding, he owes the auctioneer a certain sum, usually a percentage of the planned starting bid, as a penalty. On eBay, the seller loses his listing fee if he prematurely withdraws an item, which serves a similar purpose. Unlike a "real" auction, he can withdraw it before the auction closes without further penalty even if he's had a bid on it. If he does that too often, though, eBay will suspend his selling privileges.
I agree that it doesn't seem fair for a seller to withdraw an item that has one or more good-faith bids on it. Sometimes, as you say, it's because the seller isn't satisfied with the amount that's been bid to date. More often, though, it's because someone has contacted the seller privately and offered to buy it outright for a sum that is satisfactory to the buyer. By selling the item to a private offeror, the seller loses his listing fee but he doesn't have to pay the eBay premium on an item that's sold on eBay. Needless to say, eBay takes a dim view of this practice but how can they prove that's what happened?
I've also seen situations where a seller has mistakenly listed a valuable item for a low price because he didn't realize what he had and found out before the auction closed. Every bidder looks for deals like that, but can you really blame the seller for pulling his item out if he suddenly realizes he's going to take a bath? I'd like to hear from some sellers about that.