Imperf 1327 Sheet, Partially Stuck To Backing

I have a Russian stamp sheet, a 1948 imperf 1327(a?) Lenin death annv, that has been in storage for many decades. It is partially stuck to the backing cardboard it was purchased in by the original owner in 1950. I'm looking to sell this item, but cannot scan the gum side.
I'm assuming more damage would be done than value gained by trying to remove it. Is this assumption right? or is there a process that this can be done safely?
Note: I am a newbie and somewhat illiterate when it comes to these matters, so please speak SLOWLY.

I will speak as a philatelist and former part time archivist. Don't tamper with a document unless you are an expert and know what you are doing. If you want to sell it, do so and note the condition.

Thanx, that was my assumption, but didn't know if there was a safe way of doing so and wouldn't hurt to ask.