The eBay rules allow the seller to cancel the listing -- for any reason,
for no reason -- if he has received no bids. If a potential buyer offers
more money than the seller thinks he'll receive in an open auction,
why shouldn't he cancel the auction and sell to that buyer? Who,
exactly -- other than eBay, who made the rule in the first place --
is being cheated?

Uh, if I place a bid on and item and I find it cheaper before the auction
is over, I am legally bound by my bid. In the same way, if the seller
puts it up for auction, he is obligated to sell the item to the high bidder
even if he gets a higher bid else where.

Please reread the second line above -- "if he has received no bids."