I buy, trade and sell all the time. If a gun doesn't particularly do it for me ill sell/trade it for something else I want. I am also in the mindset that I don't need two guns for the same purpose. For example, I have a Ruger SR9C. I'd like to pick up an M&P9C in the future but it makes no sense to keep the Ruger and the M&P so the Ruger is going to be sold. Heck I'm in the process of selling my Sig 232 I picked up just a month ago. Great gun, but the fat grip on the gun takes away from the slimness of the slide and the frame, I realized I carry a compact 9mm easier than the 232 in .380. Live and learn.
As long as you buy low and sell high, you can never go wrong.
More often than not ill take a slight loss when selling or trading usually to sell it more quickly. That extra $25-50 I dropped off the price I can live without, and usually it's the difference of finding a buyer for my gun in a week or so compared to a month or more.