Hey Troy
The Equipment Exchange is where you want to go for used firearms and accessories here at CGN. It requires access to be granted, but once that's in place you're good to go (check out the rules and regulations to succeed in making access to the CGN EE happen).
Also most sellers prefer to receive payment for used firearms via an EMT or Electronic Money Transfer. It's really quite easy to do as it's just an e-mail from your bank account to the seller with the correct amount included. There can be a password required so include that when communicating with the seller and an EMT account is just a quick and easy process to set-up through any of the major banks (just ask a bank teller or go online to figure it out, as it's quite easy to do).
Cheers D