Getting best image quality out of ebay

Question

According to ebay documentation (https://developer.ebay.com/devzone/guides/features-guide/content/Development/pictures-intro.html), uploaded images must be in JPG, BMP, GIF, TIF, or PNG format, and that TIF, BMP and PNG files are converted to JPG.

When opening images of ebay items, they always open as jpgs with ".jpg" extension, however when you change the image extension at the end of URL from ".jpg" to ".png" the png image always gets produced, and the png files have better quality when doing visual comparison, futhermore gif images are also always produced when changing ".jpg" to ".gif" and I doubt that every seller is uploading their files in gif format. So I'm wondering what exactly is happening here, and my goal is to get the file in the best format, preferably original as uploaded by the seller.