This is actually a very good question, as most people make the mistake of using sandbags when sighting in an air rifle. The question is, what type of air rifle? If it's a springer, you want to hold the gun in a relaxed way. If you hold it real tight against your shoulder, it will be hard to sight in because of the unique recoil. The same goes for sandbags with a springer. If you are going to use sandbags, put your hand underneath the forestock in a relaxed way. Also, get a scope stop, so that you can stop "scope travel" which tends to occur with springers. As for PCPS, and pumps, there is no recoil, so you can pretty much sight in like you would a firearm.
check the link below and contact customer service. Crosman list that some parts are still available for this rifle.
I recommend checking online for videos and forums to assist you in setting the sight for your rifle. With each rifle and barrell, it will differ on how the sight is set and someone with more knowledge may be able to assist you.
It depends on which model you are talking about. Benjamin has made over 70 different models over the years.
Assuming you mean a Mauser 24/47, use the sight as you would any open rifle sight. The slider on the rear sight adjusts for range- marked in meters.
You sight it to a specific range , e.g. 100 meters. Clamp the rifle to a solid object, or use a benchrest. Shoot at a target and adjust the sight accordingly, until the holes in the target zero in to the sight.
Crosman still has parts for this rifle. contact Crosman at the link below (Customer service) and order the parts you need.
Should be a Stevens rifle, but the peep sight would probably be aftermarket. If you can send a digital picture of the rifle and a closeup of the sight we can probably ID it for you. sales@countrygunsmith.net
adjust rear sight up for distance
The same as you would any other rifle.
It would be considered fair condition from your description, and worth between $55 - $75. Depend on the rest of the condition of the rifle.
No
You need a gunsmith.