You just got unlucky. Someone recently vended a Katherine Keyron card for 99m and I bought it. It was ironic because she was vending it right beside the raffler, and no one got it for a few minutes!
I stood there to wait for her to close her shop to see if she'd around, pmed her about it and taught her her mistake, because the card goes for way more on the forum market. She felt really bad when I told her the actual value of the card and I halved her damage by giving her some more zeny than that.
I suppose it's very rare the buyer would come back and correct the seller's mistake, I consider her VERY lucky I got her back. =p You never know this might happen to you so please don't feel discouraged to keep playing.
I myself lost some zenies by accidentally missing a 0 before. But my damage hasn't been as severe, lost perhaps 50m or so. It definitely is a learning curve.
Edited by WarlockFier, 17 January 2014 - 08:31 AM.