Looks like it was stuck to stamps it was on top of, which would be considered damage. I've pealed a lot of stamps apart, and the result look just like that. It only takes a little moisture to make them stick together.
I'd need really close-up, high resolution, photos to be able to venture a guess about the gum. You are basically looking for clues to reguming. See:http://www.philatelicfoundation.org...en-regummed/
I doubt you'll get much on eBay
without a Cert.The question is, with a damaged stamp, will it bring in more than a cert costs?