Paid $100 for a softball bat and sold it literally within 30 minutes for $375. Bought it from a guy who calls me when he finds interesting stuff. He knew what the bat was worth, and found it at a thrift store for 2 dollars. I offered to let him consign it with me, or I could give him $100 on the spot. Apparently he needed the money. 10 minutes after posting, this guy starts hitting me with best offers. The bat was worth about $500. So the guy I got it from made $98. I made $275. The guy I sold it to probably made $100. I love it when so many people can make money from a transaction. Plenty of meat on that bone and I was cool with a big quick-flip.
Paid $150 for a lot of 6 educational/industrial robot arms. Sold one for $600. Got around $600 total for the rest of them. The expensive one was a Scorbot w/ controller. Bought it from a guy on the street downtown during a festival. His dad owned the antique shop he was in front of. College kid whose engineering dept. gave him the bots because they were getting rid of them.
Bought a Domino's Pizza light up car topper sign for $5, sold it for $100. Got it from a weekly cheap-o country auction. Didn't have the power hookup for the sign that goes from the cigarette lighter to the sign. Tried to sell it for a month without it, couldn't even get $40. One weekend, I'm at a garage sale and I see the exact same sign stuck to the side of a metal water fountain. They guy doesn't want to sell the sign, but he does sell me the power cord. I get home, hook it up, reshoot the lit-up sign on top of my car, then sell it the next day for $100 on a BIN.
One more. Bought a 70's cop light bar from someone off of a Facebook for $20. A little scuffed up but still worked. It had spent 20 years on top of a snowball stand, trying to attract thirsty people. Had it on eBay at $300 OBO for 2 months. Finally a guy from Texas contacts me. He already had built a General Lee from Dukes of Hazzard and was about to build Rosco P. Coltrain's car. He's passing through my neck of the woods on business the next week, so he swings by and gives me $160 cash for it. Not the best score ever, but a damn good one. More importantly, it reminded me how we are performing a service for people that need things. Can't wait to see the car after he's through.
Not sure these are the best, but they are the first ones I thought of.