I started one, but ended up with a local trade instead. IF you have more than the standard package with the gun, i.e. if you have extra mags, installed night sights, etc, then their automated list of gun values is not sufficient and you're better off contacting them first.
When I was looking to dump my PPS, for which I had 2 extra mags and extra floorplates to change mag capacity, their chart showed they would give me $325, but when I contacted them, they offered me $400. That was a better offer than any of my local gun stores would've given me (which is why I don't sell to them!).
The gun I was looking to trade for was listed at $399. I tried to talk them into an extra mag to make it an even trade, in gun & accessories, but they declined. I ended up getting a local offer that I found more intriguing, so I took it instead.
Generally, I think you'll do better trading or selling locally and then buying what you want from them. When you do a local sale, you'll get more $ for the gun, because they have to buy at wholesale in order to make a profit.
Now, if you don't want to go through the hassle and you're satisfied with what they offer you for the gun, then take their offer.
If you care to share what you're trading, what they're offering and what you're trading for, we might be able to help guide you to a better option or confirm that they're giving you a good deal.