Perhaps the best way to judge the actual value of a used firearm without seeing the gun in question is to visit the websites of retail stores selling used models like the one you are planning to purchase. Take a look at the sites for Williams Arms, Frontier Firearms, Ellwood Epps, Shooters Choice . . . the list goes on and on. Figure out an average of what a used model generally sells for in relatively good condition as described on those sites, then set your maximum buying price around that level. If you find that Remington 870 Express basic models (we're not talking the Desert Recon tactical models here!) are selling for about $250.00 -- $325.00 in good condition, then the one listed on the EE at $500.00 for a similar model is simply overpriced.
With regard to the "100 rounds fired" claims, it has been our experience that the vast majority of gun owners do not keep a faithful log of how many rounds they have put through their guns on any given day. This claim is generally the same as reading "highway miles only" when buying a used car. Take it with a grain of salt. NO GUN IS EVER UNFIRED! They are all test fired at the factory, just like no car is ever delivered with 0.0000 km on the odometer.
The Blue Book of Gun Values lists 100% with the disclaimer that the gun has not been sold at retail. Obviously if the gun is in the hands of a private owner, the gun has been sold.
99-98% value, according to the Blue Book, says the gun MAY HAVE been fired, but it is the 95% value that is listed as Typical for most models.
If you are curious as to the depreciation value of any gun after purchase, we urge you to pick up a copy of the Blue Book so that you will understand how price levels are set.
Two words of advice to all sellers:
1) Just because you paid a few hundred dollars for a BladeTech holster, spare magazines, snap caps, fibre optic sights and so forth does not mean that you will recover 100% of that value on a used gun. A new Glock 17 sells for $675 in most places. A used model with a few hundred (or thousand) rounds through it, with two spare magazines and a holster that sold for $75.00 new does not translate into a viable selling price of $750.00.
2) When comparing retail values on used firearms at your local gun store versus the amount you wish to receive when you trade any of your own guns in, remember that the store needs to make a profit in order to keep their doors open, therefore if you expect the gun to sell for $500.00 over the counter, you will not RECEIVE $500.00 when you bring the gun in, but rather a set percentage of that value as the store must also make a profit to keep the transaction viable.
they can keep it for all I care. There are enough realistic sellers out there you don't need to aggrivate yourself with the guys that are out to lunch. 
