I have found investing in what are called PNC’s has been very lucrative. They are stamp/coin combination sets that are released periodically. So far they seem to return 50% to 300% profit on Ebay within a month or two of the purchasing them at their release. The trick is guessing which ones will be in demand and getting enough of them to make it worth while. Then you have to patient while you sell them off slowly enough not to flood the market and lower the price.