Hi K. - well, here is my 2cents for what it's worth :)
I think if you want to teach your boys the value of money (which is great!), then something like a garage sale might be the best because they will see the immediate gain from "selling" as people give them money and they can see the money add up, they can help decide whether to take a little less if someone offers, etc. I think that's a GREAT way to involve them, help set up/clean up, work the sale, make cookies to sell, let them count the money, and deposit the money, or whatever.
eBay, on the other hand, is a terrific way to sell stuff. I've had great experiences selling. But you need to spend the time to create each auction, answer any questions from potential buyers, wait for the auction to end, make sure people pay, take the trouble to ship/insure packages, etc. It doesn't have the immediate gratification that a grarage sale does, but you could certainly earn more money!
There is always other options, which are also (IMHO) worthy i.e. donating and FreeCycle, and other trade-type sites... it's nice to talk to our kids about giving and getting nothing simply to help someone else or to give to a child who doesn't have as much. What about choosing and giving specific toys to friends that might really like to have a special toy, Children's Hospitals, preschools, your local church nursery, etc.
of course, the money is always nice to have, too ;)