I owned a handgun that held 17 rounds in the magazine. I no longer own the gun but I still own 3 magazines. I come to find out that Ebay doesn't like to let me sell anything over 10 rounds. Silly me.
That is the trick, know the law and follow it.
There are a myriad of ways to put them out there for sale, so long as you obey the laws regarding your home state and the state of any potential buyer.
Put a good price on them and sell them here. Don't try and get retail for them - there's tons of folks on gunbroker and at gunshows selling every kind of magazine ever made. Remember, stuff priced right sells!