Welcome 1stly.
2nd, depending on what you're selling you can go to the basic Classified section:
http://www.jemsite.com/forums/f28/
Just make sure you post your location, pricing and all relevant info about the item, and pics are ALWAYS a plus with buyers here. If you're selling a Jem or UV, you can post in :
http://www.jemsite.com/forums/f31/ and the same rules apply, but this forum is exclusive for Jem and UV guitars.
Aside from that, time and familliarity will improve and people will be more than happy to give you a positive feedback rating (Kind of like EB@Y) once a few deals are under your belt, just like anything, you gain trust and earn a rep.
Once again, welcome and have fun.
Bamm