> It's always going to be a bad deal for the buyer.

That's not true. Sometimes there's not a lot of demand and you pay much less than average market price.

If something is priced super low then someone might step in to arbitrage, but even with perfect knowledge in a perfectly efficient market, an arbitrager will only be willing to pay the true value minus the cost of relisting, the cost of reshipping, the cost of their time, the cost of tying up their money, and the cost of the risk it won't resell. If you beat that by fifty cents you'll get a great deal on the item.