Take Craigslist, for example. On a normal day I could find a listing for a "Complete Paintball Set" that comes with a Viewloader Orion, a hopper, basic mask and 12oz CO2 tank. That's a fine little package, but the price of $120 is really out of the ballpark. While the seller is probably basing the sale on the fact that he paid $150 for the package three years ago and only used it once, it doesn't mean anyone is ever going to pay that. In the last few years the price of new guns has dropped considerably and an equivalent or better new package today would cost less than $100. The truth is that a package like that really isn't worth more than $40-$50, and that's if the gun is really in like-new condition.
The other kind of seller is the one who invests too much money in a gun and then realizes that the $500+ in upgrades really hasn't made his Tippmann A-5 into a better paintball gun. Since he's invested around $700 in the gun and done all the work of ordering and installing the upgrades, the seller figures he can sell the gun for $600 and whoever buys it is getting a steal. Unfortunately, things don't really work like that. A well-used gun, even with lots of upgrades, is still a well-used gun. Selling an upgraded gun for half of what you put into it still might be too much.
The final area where people really get off on the wrong foot when they price their gear is when they think a high-end gun from a few years ago is still worth quite a bit. If, for example, you spent $1000 on an upgraded Impulse in 2003, you may think that this gun is still worth something. That, though, really isn't the case. While the Impulse is still a fine gun, it's no longer top-of-the-line and it shows its age. An older tournament gun is rarely worth more than $150-$200 and often much less. They still are fun to play with but they just don't warrant a high price. As it is, I'd rather buy a brand new Proto SLG than spend $200 on a five-year old Impulse, and I'm sure that most players would agree.
The general rule that I've found is that used paintball guns simply aren't worth what you wish they were. If you want to make a good investment, buy real estate. Don't figure that you'll be able to get anything near what you paid new when you sell your paintball gear used.
Before you sell, do yourself a favor - take a look on eBay or browse through the forums and find out what things are actually going for. Maybe that way you'll actually be able to make a sale.