I always seem to get burned when I list something as an auction rather than a Buy-it-Now. I was just looking at recent sales of John Madden Football Championship Edition for the Genesis, and they've been consistently selling for $100-$150.
Two months ago, I decided to sell one of mine. I usually look at the most recent sales and come up with a reasonable Buy-it-Now price based on that data. They were selling for around $80 complete back then. So, I started my auction at $40 and figured I'd generate some interest (and hopefully a bidding war) because mine was absolutely mint - one of the best examples you'll ever see. I let it run for a week, and it ended up at only $44.
So, apparently in the weeks before my auction people were gladly paying $80+, and in the weeks following my auction they were gladly paying $80+, but in the week that mine was listed nobody though it was worth more than $45. Now, it's suddenly skyrocketed. Luckily, I have another one that's in just as good condition, but it still irks me.
Similar thing happened a few months ago when I sold a Paper Mario game for N64. Loose cartridges were averaging around $30, and I had one in the original box. I decided to auction it, hoping for some action, and ended up getting $30.01 for it.
Oh well, I guess I could look at the bright side. Now that the Madden has taken off, my collection is a little more valuable.