Stamps are now such a small fraction of
ebay's sales, it doesn't matter what any other stamp web site does. It won't affect
ebay's actions or bottom line.
As for
Amazon, I have seen stamps sold on the site in the past, but not recently. From what I've read,
Amazon is more draconian than
ebay in controlling third part sales. Delcampe and Bidstart are the two biggest stamp "competitors" to
ebay and the first one does have unlimited free listings technically.