For the most part, guns are a break even at best investment.
On the rare occasion I find a nice gun at a good deal, its usually something I decide never to sell.
Most of the time, its value only increases based on inflation, or maybe slightly higher. I'll never own a Purdey shotgun, or a Holland & Holland double rifle, so that isn't even worth bringing up. Yes, they are likely to be better investments, but its also a niche market.
I've been seeing a lot of 'like new, less than XX rounds through it' guns on the online gun classifieds. Most of them are still asking new retail, or even higher. A S&W M&P sport, with two mags brand new in the box retails today for under $700, so why would I buy a used one, taking the sellers word that its only had 60 rounds through it, for his asking price of $750?
Most guns I've sold, I've done so at a loss, or at best, broke even.