I buy and sell select car parts on Ebay, Car forums, and Craigslist.
Just a few weeks back, bought some wheels for $400 locally, sold them for $675. Last week, I bought a windscreen for a convertible for $150, two days later sold it for $275.
I typically get a 50-100% return on my investment. The typical hold time on inventory can be a week to a few months. Very easy to do if you have knowledge of a specific market or item and some cash to play with.