I've purchased one handgun on there and a couple of knives. The handgun came from a dealer in Texas. Just look for their feedback ratings before comitting to a purchase. Search for what you're wanting, then search locally for the same thing...money says you'll find the same exact thing locally (or can have a FFL holder order it for you) for less than it would cost after shipping and FFL fees. As with all online sales, there is always a chance you can be scammed (we've had scammers on the classified section here).

To bid on something, sign up on the site for an account, find what you're wanting, then enter your bid. If there are other bids higher than yours, you'll get an email stating that you've been outbid or you're not the highest bidder. I don't know if it's still the same as it was last time I used the site, but if someone places the highest bid in the last 15 minutes of the listing, the end time is automatically extended another 15 minutes (think thats what it was). If you are the highest bidder, you send payment to the seller and your FFL transfer dealer information (many FFL holders will want to send it themselves). Seller sends the firearm to your FFL, who will perform the NICS check on you. You clear the background check, pay the dealer their FFL fee, and firearm is transferred to you.

https://support.gunbroker.com/hc/en-us/articles/226246808-How-to-Bid-and-Buy