>Why not just set the minimum bid at the lowest price you’d be willing to sell at

Because if the buyers know you'd be willing to sell it for [x low price], there's less reason for them to bid any higher than that.

Even if you get lucky as a seller and there's a bidding war, if all the sellers know you'd be willing to sell for $100, odds are nobody's going to bid $1000. If the "lowest price you'd be willing to sell for at all" is a mystery to the bidders, it encourages higher bids.