Generally, I tend to gravitate toward the community sites/forums for my gear. So, if I want to sell my micro four thirds gear, I'd start by putting it up for sell on the mu-43.com forum. Good communities out there, be a part of them!
If it's Canon or related gear, the Canon fan site, Photography-On-The-Net (POTN) has an excellent sales forum. I use it often.
However, I believe there are restrictions on new members selling stuff; there may be a waiting period.