A model C .20 caliber not a model C9 or C9PB in very good condition is worth around $105 to $130. If you want the owners manual, see the link below. Download the C9 owners manual it will be almost the same as the "C"
Crosman airguns owns both Sheridan and Benjamin airguns.
I have seen these sell from $50 to $600 depending on model and condition. Most are in the $90 - 100 range. Have a look at www.proxibid.com (sheridan pellet) and search the archives for 2008, it should give you a better idea what you gun is worth.
25-75 USD
It depends on the model, wight of the pellet, shape of the pellet, length of the barrel, Power source, IE Pump, C02, Spring or PreCharged Pneumatic. (PCP) All these play a factor in the length of the shot. Anywhere from a few hundred feet to 1/2 a mile.
I believe you are asking about a Sheridan .20 cal (5mm) air rifle. They started to make this rifle in 1949 and stopped in 1990 ( sheridan sliverstreak model ) During the 1990's the name changed to C9 sliverstreak and C9 bluestreak. The value is determined by the current condition of the rifle. You did not give any such information , therefor I can not answer you.
It all depends on the power source but PCP (PreCharged Pneumatic ) air rifles will give you the furthest range depending on the shape and weight of the pellet. Light pellets will travel further and heaver pellets have more knock down power and will fly straighter.
Around $50
the size and shape
It would be worth around $65 in Excellent shape.
Can not be answered without knowing how heavy the pellet is, how fast it will be leaving the muzzle, pellet shape, location where it will be shot.
Pellet grain is really just the type of shape of the grain. Instead of flake grain it is just shaped like a pellet or, if you will, like a perscriptian drug.
They can be punctured, letting the air out on which they rely to give them their shape and cushioning.
Velocity and shape of projectiles.