to clarify, main reason for stamping raers is actually security, no you cannot prevent your precious ones from getting stolen, stories bout records getting snatched are getting more and more common these days, and even though marked records would not scare a record thief away, he wouldnt be able to remove a stamp/ name evidence, odds would become greater that records could be traced to the orig owner in a theft scenario, if he was gonna try to sell off the merchandise..
regarding ruining record condition/ value by stamp marking, for sure is an issue if you place the stamp on the front or back cover, but if you reach one/ two inches inside the sleeve, a stamp will not be noticeable (except if you look deep into the sleeve) and i dont see that as messing with the condition or decreasing market value of an album.. just sayin!