Can you see the sand or dirt when the screen is turned off?
If so, you've just got some dirt between the screen and the faceplate, which is easily cleaned by removing the faceplate and wiping it and the screen with a cloth (there are screws on a few places, and notably, beneath the warranty sticker in the battery enclosure). Though, of course, this voids your warranty, but if you're selling it I doubt it should matter much anyway.
The scratches will be on the faceplate, which can be replaced altogether if you want an immaculate surface. Otherwise, there's no real way to get rid of scratches on a faceplate.
If you remove the faceplate and clean the screen itself, and you can still see the messed-up pixels, then you'll need to replace the screen, which can cost around $100. If this is the case, you're better off just selling the PSP with the messed up pixels, and cleaning up faceplate to make it look a little better.
