There are three things that come to mind when I think "I really should've bought that..."
One was a copy of Conkers Bad Fur Day for $30, I had no idea what this game was when I saw it, all I thought was "What Nintendo 64 game costs thirty bucks?!" O.o
The second is an Odessey 2 for $10, I didn't know I if it worked, but thinking back, it would've been worth the gamble :/
The third was a boxed colecovison for $25, this was missing the power adapter but this was before I knew about Amazon and EBay, so this too would have been a gamble, but man...I wish I got it. :/