Pinworms or protozoa are often a culprit in this. The extremely small size of the bowel movement that caused this would seem to be more indicative of that than him being previously overfed, but who knows. Hopefully his meds will clear it up.
I'm not trying be rude... But, are you a vet? You are stating your opinion.... Which no one gives two turds about. We want facts. Facts about the issue at hand... Not what you think may have caused it or what your opinion is about the seller. The BOI is a place for facts... People misconstrue that so much.
You know what? We get it... You aren't happy. Now keep those thoughts to yourself and lets get on with the facts.
For instance. It's a fact that once an animal prolapses that there is a VERY high chance that it will happen again. You knew the animal had prolapsed once... It seems you also knew that the animal may arrive with mites. Neither of these issues stopped you from purchasing the animal. So please explain how that's the sellers fault? You purchased the animal willingly, knowing all these things. Your fault[PERIOD]
You should have done what he offered and just had the animal replaced. Once the animal isn't in your care it is no longer your problem to worry about. You seem to be CHOOSING to keep the animal, which means you must want to be responsible for the upcoming vet bills to boot.
By all means if I am wrong about any of this I can assure you I'll be the first to apologize.... but from what I've read and seen, this is spot on.
BTW - I hope that little retic pulls through!