Obviously the animal is a picky feeder and while it lost weight all that might be relative and there is no way to prove whether the animal lost weight because he was not feeding with seller A or because of something else, depending on whether the animal was bred and are heavily he was bred that kind of weight loss is possible (as an example I had a male losing 200 grams during breeding season and doing so every season while other maintained their weight)

So as far as it goes it would be hard to prove anything or get any type of compensation, you are not out of anything you just need to find way to get him to eat, remember when you buy an adult even one that feed for you that does not mean you will be able to use him in your projects right away, in my experience it's been more of a 50/50 (50% going into the projects right away, 50% stop eating on arrival and take 6 months to a year to adjust). You are dealing with live animals they go on their schedule not yours.

Now as far as the animal not eating for you, it's something you run into a lot more with mature animals, they seem to take longer to adjust, if you don't already have him in a V18 or 15 quarts tub with aspen or coco coir chips and an hide.

And green in bowel movement is due to bile, it's not necessary because an animal is starving, the opposite is true to.