As my friend, oneounceload, astutely observed, why would you want to sell your comp Skeet gun? You put a lot of time and effort into getting it just like you want it. Usually the only comp gun you'd sell is a mid-grade one when you move up to a higher grade unit. But, many folks keep that gun as a back-up or loaner. Of course, when a new model is released, like the K-80 or the Kolar there are some folks who'll sell off their old guns to get the latest offering. They hope the new gun will shoot higher scores. These are the folks who's comp guns are often unaltered from the factory's offering, and they may trade them in when changing guns.
Put yourself in the buyers position, what's he looking for in a used Skeet gun? The typically answer is, he's looking for a bargain in a gun that has a lot of life remaining. Don't expect to get full value from a tube set. Smaller tidily-bits, like trigger jobs and upgraded sight beads will probably add little to nothing to the value of the gun.
An exception to the rule is the shooter who's looking for a variety that's been discontinued. Remington no longer makes Skeet grade 1100s, the Beretta auto-loaders captured their market. But, there's probably someone out there who wants an 1100 Skeet, and he may be willing to pay a little extra to get one.
When individualizing a comp gun, make sure you don't do any bizarre mods that would preclude anyone else wanting it. I'm thinking of one shooter who liked his butt plate significantly canted. He didn't have an adjustable butt plate installed on his new K-80. No, he had a JS Air Cushion stock fabricated with the twist built-in. With that bizarre twist, a buyer is going to deduct the price of a new stock from his offer. However, a conventional JS Air Cushion would be a premium to most buyers.
With higher grade guns, the price may vary significantly from gun to gun. Two otherwise equal guns, that had the same original price, may get totally different offers because of the wood. We've all seen shotguns that have have much nicer, or poor looking wood than what's average for that model.
Sometimes a seller is "motivated" and a gun is just too good to be true. I picked up a un-shot P-gun that had been significantly upgraded by a LGS (gorgeous new wood, custom engraving, super-blue, Tula chokes, custom tube set, and gold plated internal parts). The LGS owner needed quick cash and I got the gun for less that the price of the new gun and tubes. My buddy, who shots the same number P-gun stock as I do, went with me to check out the gun. He put it to his shoulder and said, "I sure hope you buy it. Because if you don't, then I gotta, and I don't have the money right now." I've been offered $5K more than I paid for it. Hopefully you won't find yourself in that seller's position and be required to sell your gun at a significant loss.
Another factor is the economy in general. With unemployment at virtually 10% and an unstable market, this is not the best time to be selling a comp gun.