+1 on supporting BPL. It allows reaching the exact users you want and is far faster and easier than eBay.
If you live in a big city with a lot of hikers, Craigslist is a viable option, but nowhere near the concentration of users as BPL and tends to be more seasonal. Craigslist can be good for more traditional gear.
eBay does work, but not as well for the more esoteric gear. If you have mainstream branded gear, eBay is fine, but it is on a case-by-case basis. You may find some items flooded with listings (dropping the price) and you may need to take more time to educate your buyers with more information and detail in the listings, better photos, etc.
You may find local hiking web site with a gear trade forum. NWHikers.net is a good example for the Pacific NW. There are other national sites, but most require some level of membership like BPL and Hammock Forums. No free lunches!