Hard to say what my best is, found a Stanley number one hand plane about three years ago in the box like brand-new, sold it for $1200. Paid five bucks. Purchased a set of identical rolltop desks about two months ago for $500, sold them for $1500 each. I have purchased all sorts of vintage handtools, antique furniture, musical instruments, and other things at fleamarket's. It's not really about getting a great price, it's about knowing every detail and complete history of what you are purchasing, understanding the market of that item and how saturated it may or may not be at the time, and most importantly how to sell them for the maximum amount of cash that they could possibly ever be worth.
You win win some you lose some. Nobody ever talks about the things that buy that they never make money from lol. It's kind of like playing the lottery, you only hear about it when people win but it takes a whole lot of money to get to that point