I bought a box lot a couple of days back and -- you guessed -- the collector had used sellotape to mount some mint GB.
Sellotape usually leaves terrible residue on used stamps, soaks right through the stamp and discolours the page as well, but in this lot the gum seems to have stopped the solvent from entering the paper.
Luckily I am only using the stamps for postage anyway, so its no great problem, just a nuisance to have to peel the stuff so slowly and to keep the pages as is, until ready for use.
Some modern "sellotape" (aka "cellophane" tape in the U.S. or more commonly just "Scotch tape," a brand name) has less aggressive adhesive and is easier to peel off. It used to be sold as Scotch "magic" tape though I haven't kept up with the world or tape much in recent years. The adhesive on older scotch tape really did soak into anything it was attached to, leaving a gummy brown residue. Awful. And to think I was one of the young collectors responsible for doing that. I blame it on lack of responsible adult leadership.