lots of places from here, to your local news paper, tons of on line gun sites and forums, the sky is the limit on where you can advertise a gun for sale these days!

start with basic's when posting add, like make model, age if older, condition, location your at, and asking price, along with some basic pictures and any details on add on's you added to gun

from there let possible buyers ask questions of more details!
also make sure you know the LEGAL rules of selling a firearm in your state as well as federal laws

so as not to get you in legal trouble!