The best way to come out well if you sell your guns is ...
1. Buy good guns that are easy to resell. Ruger, Colt, S&W, Remington are guns that you'll get a lot of calls on if you run an ad in the local paper. Buy guns that you know up-front will be easy to resell if you want or need to (1911's, GP100's, 686's and 700's are easy to sell). Avoid unusual models and avoid calibers that most people aren't comfortable with or that only a reloader would want.
2. Buy your guns USED in excellent condition and try to sell them in similar condition (TAKE CARE OF YOUR GUNS). This avoids taking the hit on depreciation between a new gun and a used one, sorta like buying or selling a car. When you're selling, condition is everything!
3. Don't leave all of your goodies on a gun or sell you gun with all of the accessories. You can come out better taking the scope you paid $500.00 for and keeping it for something else or putting it up for sale on Ebay! Everybody expects you to just "throw-in" all of the accessories for free no matter how much you've invested!!