If 'average price' only applied to new products, there wouldn't be car blue books and various other such guides. Pawn shops and gun shops also have a good idea what they can buy & sell used items for. Yes, there are outlier buyers and sellers. Some guys will pay a lot more for something than most. Some will list something far higher than what most will pay (see that on GB all the time). If those two meet, both will be satisfied. Its far more likely that the ad will get no replies for a very long time.

Example - If the average price in a given area is $ x for a gun in y condition and you paid 50% more than that, you may be satisfied, but you still didn't get a good deal. And by the way, there have been times when I paid more for a gun than I should have. I'd been looking a long time and didnt want to spend the time & effort looking further. Other times, the gun just looked so darn 'purdy' that I had to have it !
I've paid a premium above the existing market for top condition guns. And actually, over time that has worked out well with rising prices.

P.S. - there are statistical methods for accounting for outliers.