When offering an item in on-line auctions (eBay, GunBroker, etc) I try to set the minimum bid at the least amount I am willing to accept. On collectibles and other very desirable items I will set a "Buy Now" price close to the maximum realistic value of the item, allowing those who really want one to act on it and secure the deal. This approach has worked for me consistently as long as I have been dealing on-line.
Every item has a range of value depending on condition, rarity, and other factors. There is usually someone looking for just about every item. Auctions, in general, allow potential purchasers to establish the value of any item up for sale. The key for sellers is to allow a way for the genuinely motivated buyer to get what he wants or needs, hence the "Buy Now" price offer.
As a seller I have experienced several incidents of bidding exceeding the "Buy Now" price offered, occasionally by a wide margin. As a buyer I have been tempted by "Buy Now" prices, but waited to try and snipe a deal toward the last few minutes, only to see others bid the item up beyond what I was willing to pay.