A "slightly used" gun, properly cared for, is worth no less than a brand-new, unfired gun, IMO. It might even be worth more; some AR's act up when new, need to be "worn in" from what I've seen in many of them, so if they become more reliable after having a couple hundred rounds through the pipe, I'd say it's worth more than a new, troublesome gun. If I bought a gun in a "panic buy" and paid too much for it, I'm sure not going to take a loss and sell it because I don't need it or want it. Hang onto it long enough, it will be worth what you paid for it (especially during the next panic).
Judging from the prices at the gun show last week, the older the gun, within limits, the more it's worth, at least to the seller. I bought an old SxS shotgun, hasn't been made since the '80s, and the guy wanted twice what it sold new for. (I didn't pay that much, though). Through some fierce dickering and a trade that went my way in value, I got the gun for just a bit less than its market value.
To me, a gun, whether it's a handgun, shotgun, or rifle, does not decrease in value unless it is seriously abused, neglected (in care and maintenance), or has just had the action and barrel shot out of it.