My best purchase was from eBay about 15 years ago. I bought an autographed hat, signed by the entire 1975-1976 Philadelphia Flyers for $12. I had the sigs authenticated and they assigned a value of about $1200 and that was way back in 2001.
I bought some gravity boots and a door frame pole from a flea market for $20 ... I absolutely loved hanging upside down cos I did ... But I always got stuck and hurt cos I had to fall out of them ... It was a lesson though ," don't get your self in situations you have no bloody idea how to get out of " 20 dollars well spent :)