Before I got a decent knowledge of the hobby, I sold a rare version of Kiss' first record for $5, and it's worth more like $25. Only then, I think it was fetching $50, because Kiss announced their Farewell Tour, and at the time, it looked like they were serious.
And, I passed on Pearl Jam's Binaural album for $10. It's now fetching $50 because it's out-of-print, as is Live on Two Legs. Oooof.