My story is simultaneously one of the best scores I got on eBay, and also the best score someone got off me. A few years ago, I found a Magic the Gathering Collector's Edition boxed set for sale on Ebay for $40. It was worth around $1,500 at the time. This set was complete, and every card was near mint. It was sold by someone who obviously resells stuff from flea markets, garage sales, estate sales, etc., since they had all kinds of random stuff up for sale. Score!
I really liked this set. It is comprised of a complete reprinted set of Magic the Gathering Beta cards, but with square corners and a gold border back. I kinda wanted to keep it long-term for the nostalgia, since I played MTG for years. However, the cards weren't actually tournament playable and I figured I better cash in now before MTG loses popularity and the cards lose value. I ended up re-selling the set for around $1,300. Huge win!
Except for the fact that this set is now worth around $8,000 - $11,000.