bvol, perhaps I can shed some light on our sensitivity to the details of this gun since as per your admission, you aren't a "shotgun guy." I'm not a black guns guy so I would be in your shoes if I were ever to try selling one.
The 870 Competition was one of the holy grails of the 870 line. Only about 5,000 were made over a five-year period in the mid-1980s and they were unique in many ways. As you probably know, they are a single-shot piece with a gas-assisted recoil reduction system and the ONLY such 870 ever made. Their stocks were shaped in a way unique to that model - one of the gun's intents was long-yardage handicap shooting, so the comb is actually higher than a regular 870 trap gun's, even ones with a Monte Carlo stock (which really aren't any higher than a straight-comb stock). The wrist of the stock is shaped differently than any other 870's. Their barrels were considered some of Remington's best, if not their VERY best and before any knowledgeable 870 Comp buyer is going to consider your gun, he will want bore micrometer readings. Once those barrels are altered, collector value is gone. They were overbored fixed-choke barrels with .036" to .040" of constriction. Their trigger groups were hand-assembled and tuned by just two men in the Remington factory and they engraved their employee numbers in the trigger housing.
Those guns in like-new unaltered condition will fetch $800 these days. Your asking price is not out of line for the gun's condition IF the answers to the necessary questions are correct (bore measurements, stock measurements, etc.). Otherwise, it's worth a LOT less and that's why the stock questions are so pointed. Those are collector items these days and originality is very important. If that stock's extension can be removed and leave the gun with a factory length of pull, that's good, If not, it isn't.
I agree with Ed. Additionally this particular gun looks to have had the stock cut and then reglued together. The grain is a perfect match and the LOP is right. It also has a lot of corrosion around the gas port and likely inside as well. The gas seal isn't original and looks kind of big. This is a useable gun but has been monkeyed with and has been somewhat neglected. I still appreciate you posting it here. It does have value to the right buyer.