I don't believe in having lots and lots of guns unless you're an infantry battalion or a serious collector. You need someplace secure to store them; you have to insure them; and they represent money that's tied up doing nothing. So periodically you must cull the herd. The question is, which guns go down the road? Here are some things I've learned the hard way.
Don'tsell any gun that was given to you, or which has real sentimental value, or which holds a lot of memories. You'll get the money for it, realize what you've done, and open your veins.
Don't sell any gun that is generally useful. If you have a .270, for example, there's very little you can't hunt with it. Hang onto it. On the other hand, it's OK to let go of your .700 Thunderf***er unless you plan to go to Africa soon.
Do sell any gun that you have not shot, or hunted with, for a year or more. If it sits in the safe for long periods of time, you probably won't miss it.
Do sell any gun that has given you fits, and into which you have poured money, but which still does not quite work. However, when you part with it, inform the dealer of its history. To do less than this invites permanent bad blood.
As your interests and circumstances change, so will your gun collection. I've recently put up for sale a .480 Ruger Super Redhawk because arthritis is claiming my hand and the recoil is too much. Why keep it? Similarly, I'm parting with a lovely 1970s Colt Python because I used it for shooting 50-yard bull's-eye and I'm no longer competitive at that. Why keep it?
There are some guns which, for mysterious reasons, we do not shoot well. Sell them. Keep the ones you do shoot well.
If you have to choose between two guns, part with the one that kicks harder.
Don't put outlandish prices on your guns. Get a copy of Fjestad's Blue Book of Gun Values, go online, and talk with your dealer to get an idea of what price will move them. Give the dealer the prerogative to lower the price if he thinks he's close to a sale.
And if you make a mistake, don't worry; you can always slit your wrists.