Remember that their evaluation can be much above your purchase price.
For example, that wizard DID pay 222 gold for the stuff that you bought at 200 gold. He was actually willing to pay more, something around 240 gold. So you can still make a profit there.
The problem right now is if he already owns a Staff of Fireball (200 gold) and you want to sell him a (hypothetical) Staff of Fireball +1 (300 gold). He will think his existing staff is worth around 100 gold, and the +1 might be worth 390 to him, but because of that he'll only pay 290 - a loss for you.
