What should I sell my pony for?
Value would vary widely by location and what demand for such a pony would be in your area. Not many people in my area are in the market for such a small pony, let alone one that's been ridden "HARD", "countless" times, at only four years old- a good recipe for joint problems later in life.
It's a gelding. It's worthless.
I am glad I don't know where you are, because sometimes I am a sucker for "rescue" cases.
find him a nice owner who will love him and don't treat your animal like a commodity. jeez, poor creature, find it a home where it doesn't have to go on HARD rides and is just loved.