Post a list of all the items you have for sale on a bulletin board in the gym - there are always plenty of gym items for sale on our bulletin board in the parents' observation area. Also, put an ad in the Booster club newsletter. And of course, leos sell FAST on ebay.
Check with the gym to see if they have any type of used leo/gym stuff sales during the year. If they don't I would ask before posting a list of stuff for sale. As Bog mentioned you might start a used leo sale event for the booster club to hold. Our gym does them 2x/year(fall/spring). They take all used leos and other gym related clothes and price all the leos at $10. The gymnast selling the leo gets $6 and the booster club gets $4. They usually sell most of the leos they get.
I have never sold on Ebay but wish I could learn fast so I could dump my stuff there! I have bought several things off ebay and always had good deals and good experiences with leo...never been stiffed on a leo yet!
Our gym does not do used leo sales, but you may want to check with your gym or post the items someplace in there if allowed.