I cannot speak for everyone, but among collectors of pistols like Lugers, P.38s, and Mauser C96s the concern is disputes over ownership. There are literally millions of Lugers, but all military contract Luger have a four digit serial number. Put up a picture of a Luger showing its four digit serial number and there may be several people claiming the guns as one stolen from them; and they have receipts to prove it.

Life is messy enough without making it messier :-(