Quality, character, design, functionality, experience,
all the above,
I am not familiar with your work, but those are some things to take into account, as well as the fact that most Ebay buyers are looking for a deal , trying to pay the least amount for something , so you really need to have high demand for your product
many people on Ebay sell nice puppets, many people do not,
some are highly skilled, some are not,
some have a reputation for being great sellers, with a great product some do not,
some have extensive resumes to back up their work, others do not,
the only work of your I have seen good photos of is your Kermit, and while he is very nice, their are rough spots, here and their ,( I wont get in to a critique here)
your age says your fifteen, your doing fine work, keep it up, but you still have some things to learn, and their is always room for improvement ( myself and the other professionals here are constantly working to improve our work),
this is not meant as a slam , or anything just some thoughts and observations, keeping at the puppet building , Rome, like any artist, was not built in a day