Back in the day I traded a couple dual lands for a Shivan Dragon, a Royal Assassin, and I think another uncommon card or two, which was not a bad trade at the time (in terms of contemporary listed values I think I got the better side). A couple months back I learned that the dual lands are "worth" hundreds each and I could get everything I got in that trade for under $10.
While we referenced magazine listed prices to get a sense of comparative value, I certainly didn't realize I was making a major financial decision at the time.