I have several old license plates and am not sure what to do with them. One of them is a plate that is no longer in use (ie the state switched to different plates) but the other two are still the same as the ones in use. I don't want to just throw them away- is it illegal to sell them at a flea market or on eBay?

As you can see from this link, there are a couple of people selling them on eBay : eBay - license plates, Transportation, Parts Accessories items on eBay.com

However, if I were you I would get in contact with someone who has authority over these things, such as your local Department of Motor Vehicles to be sure that selling them is legal. :)

Good answer above!