Sounds like it's been a bit abused, in quite a dirty environment. It sounds like a lot of dust or dirt has got underneath the keys and into the trackball.
If you're capable (or have a friend who is) of pulling things apart carefully, they are quite easy to disassemble and clean. Check Youtube for videos first to see what you're up against.
The trackball will likely just need dust and fluff removed, and a lube with a small amount of spray grease.
The keys will likely just need dusting with a small paint brush or similar.
I don't know if you're keen, but best of luck either way.