I've bought and sold a lot of firearms, as I like to try things out. In general, you're going to lose around 25% of the value of a firearm from new to used... even if you've only put one round down the pipe. Again, that's a generalization though and you can't place absolutes on any of this. It's kind of like asking "do cars hold their value?" Some do, some don't.
I generally break even or make a couple $$ on my purchases. To do this, I look for "like new" condition used firearms and/or purchase primarily off Gunbroker. And, if you buy something either so common everyone has them or so "odd" that no one wants it, don't expect to get people flocking to your door.
I never buy firearms with the intention of selling them, but I often buy firearms knowing I MAY sell them at some point.