The only gun trade I ever made was my Kimber .45 which was practically brand new, for a Rem 700 in .308. It was deer season and I had lost my .270 Winchester Mod 70 rifle to a lying SOS game warden (long story) so was in the market for one when I seen his add. After the trade I took it hunting and dropped a doe with it but the scope was way off and would not stay zeroed, not to mention that it was a "tactical" and was heavy as heck. The kid contacted me and asked if I would mind trading back, I happily obliged, apparently his friend had talked him into thinking he was on the wrong end of the deal. To this day I think I was on the short end and am happy to have my Kimber back, it is one nice shooting .45. I finally made a trip to Whittaker's and came home with a Rem 700 in .308 and a Weatherby Vanguard in .270 for what of would of cost me to replace my Kimber.
When I have bartered for other things, I usually try and make it a good deal. But its between the two parties and what they feel the worth of the trade is. Once the deal is made, then its a done deal even if you find out later it was not as good as you thought it was. Now deception is another thing entirely. But a deal done in good faith should be honored when at all possible, if the gun was truly in good shape when you traded then so be it, but if there is any question about its condition then a redress may be in order.