Forums, and social networking (facebook, Twitter, etc.). Followed by eBay, Etsy, Gunbroker, etc. I'd say the best way to draw attention (which is step one) would be participation on forums and on social networks. I'd also exercise discretion on both. Most gun-related forums are hypersensitive to actual selling within the discussions. Make your presence known, but don't push product unless it's acceptable in the rules.
As for eBay, Etsy, etc., make certain you're aware of the fees--both at the front-end, and the final value kiss at the end. eBay will nail you front to back, and PayPal gets a piece, too. But, eBay has a huge number of potential buyers.