I am really not sure of what is going on in the רמב''ם and in רב שך. The way I understood the רמב''ם ה' מכירה כ:י''ד is when there is an exchange of animals and one is found dead that the seller of that animal has to bring the proof. That is different than in רמב''ם ה' מכירה כ:ט''ו in which an animal is found טרף  in which case the buyer has to bring a proof.  I thought and I think the reason for the difference is חזקת הגוף. The animal had חזקת כשרות until the last possible moment. In the case the גמרא brings in כתובות ע''ו י''ב the bride was found with a blemish and the father has to bring a proof it happened after the engagement. So there we do not go by חזקת כשרות and חזקת הגוף. Nor in the exchange of two animals do we go by חזקת הגוף that it was alive until the last possible minute. Perhaps it is possible to suggest a a reason for the רמב''ם  that is based on תוספות in נידה ב:ב that חזקת השתא cancels חזקה מעיקרא unless some other חזקה comes along to strengthen the חזקה מעיקרא? But then why would that not work for the טבח In ה' מיכרה כ:ט''ו