Blitzn, what are the recommended products you encountered in your research? Might as well share the knowledge to help future sellers.
Personally, I let ponies dry for at LEAST two months before I even list them for sale. The obviously dirty ones are beheaded and cleaned out and left apart to dry as you've described. If there's no reason to remove the head, though, I won't. I just get the surface dirt and any stray marks, and pretty up the hair. If I'm not planning on keeping the pony, I'm not going to try any risky cleaning methods, either.