Many times Tibet covers are offered which look too nice to be genuine.
Well, they can be philatelic covers with genuine stamps and genuine postmarks but in most cases they are just plain forgeries with fake stamps and fake postmarks.
If you see covers which look like the below shown ones, please stay away from them, they are not worth the paper they are printed on or the ebay listing fee.

Also, in my 25 years experience in Tibet Philately, every Tibet cover offered “as is” is fake.