I've bought and sold quite a few things on POTN. You need to be a member (which is free.) If you want to sell something, there's a minimum number of posts required (iirc 30) and a minimum duration before you're allowed to list it.
I've also been active on Astromart (commented on above.) The minimum fee is $15 per year which allows you to browse the listings and make (iirc) 10 so listings but if you don't pay the fee, you don't get to browse. Astromart has sections for digital cameras, lenses, binoculars, telescopes, eyepieces and several other categories. On the whole, there are very few cameras or lenses offered here.