What is the biggest return on investment you have experienced on a single item ?

Don't know if this was my best one, but it's right up there.

A 5 x 10 had a mattress in ok shape and some piece of biz machine on the floor facing away from us..it was about 3' x 2' x 1.5' high. No logo showing from the back. Couldn't tell what it was.

Guy bid $10; I bid $20 and got it.  The biz item was a "12-up" biz card cutter. Print shops print out biz cards either 8 up or 12 up and this machine cuts the cards in two directions separating all the cards (12 at a time) and they fall into bins.

Sold it on eBay for $1600...an 80 to 1 payoff and at high value....not like getting $80 for a $1 investment. Yep I was a happy camper. (gave the mattress to someone...not the guy who first bid !)