I've done it to make quick bucks, but not near what 'boom' does. Usually I am just business smart and know how to make it on investments.
Examples?
Bought 4 Unlimited Black Lotus M:TG cards for $450 each. Sold them 2 months ago for around $700-900 a piece (profit = $1,339.39 on the deal - I remember that figure =D).
Bought a Jerry Rice card (un-graded). Sent it in to Beckett and it came back as a 9.5. That just doesn't happen with Jerry Rice rookie cards. Sold it for $321.91 (bought for $79.99 via Buy it Now). So you just have to know what you're doing.
But people will buy anything on Ebay. IN ALL SERIOUSNESS, you could take a D size battery, burn it, and then put: "BATTERY recovered from flashlight in Doom 3!" Then put a description about how you warped to Hell one night and found that. Someone would probably put $45 on that and frame it. Seriously, people are stupid. Have fun and be safe! Ebay can be a dangerous place lol.