It's a difficult one, if you mark them, they are de-valued but how else do you prove ownership, unless they are all found together.
Good luck in getting them back.
We used to write on ours many moons ago, when they were sooooo much cheaper.
So if anyone comes across any with Cola on, they were once mine... (not stolen, just sold without my permission)...
Some prat I went out with wrote his name on my records...