I think a good analogy when dealing with Track is to think of the transaction like buying a new car, with a trade-in. Will you get more money for the car you want to trade-in by selling it yourself? Of course. But you have to deal with all that goes with it. Taking pictures, listing the car, talking to folks on the phone, email exchange, dealing with the tire kickers, etc. If you trade in the car, you won't get as much, but you also don't have to deal with all the things I just listed, and more.
I've always taken the approach to REALISTICALLY determine what the guns worth, subtract the 33%, and use that as my base for an asking price. Do I get less for the piece by using Track? Yep. But, all I have to do is ship it to them. I think they do a nice job with the writeup, take decent pictures, etc., and I've never had an issue with them, ever. So the analogy goes, if I want to sell a piece rather easily, I sometimes use Tracks services, realizing that like a car trade-in, I won't get the "full" value.
There's other sites / services available to be sure, and I've used them with good experience. I've also sold my fair share of guns on my own. For me, it just comes down to ease. I don't sell a lot of guns through Track anymore, but I've found it to be a a fairly simple process.
Ed