Hagar,
I would give almost anything to have my grandfathers rifle. That would be so cool. But if you are going to sell it yes at lest get a fair deal. And you have touched on just what drives the price. The condition is it by far. Some special features may at time be very important but condition is number one.
Your grandfathers rifle was manufactured in 1922. I have original catalogs from then and he probably spent a touch less than $25 on it. Since you have registerd it would help greatly if we could look at some photos. Take the rifle out doors into the shade and do not use the camera flash. Trust me the pictures will be better. Take a series of close ups of each side of the rifle. From that we can narrow down a range of values. Right now to little is known to be very accurate.
I look forward to some photos or at least more of a description. A standard Sporting rifle will look like the photo below. It could have an octagonal or round barrel. Probably octagonal. If it is a carbine it will have a round barrel but a steel band all the way around the barrel and fore end.
Good luck
2bit