While photographic copyright is complex, and perhaps many of these have lapsed and are now in public domain, I would question reproducing them for others without at the least the written release of the person who originally granted you the authority to copy them.  (There is the nasty matter of whether or not they had the rights that they transferred....)  Beyond the legality, I would have an issue with the ethics of reproducing these, especially for sale, without signed authorization. Indeed, I would insist on the authorization even if it were just for purposes of exhibition or other one-time use.
 
Having had much to do with small museums I know that their practices range all over the map. With this is mind I would hope that in your copying of them you would protect the original from the light of the scanner, recognize that the digital product has an unknown shelf life and consider - if you are going to serve as the repository for these - making a reproduction the old fashioned way.
 
When I was at an institution where we had occasion to copy photographs from private collections (indeed I solicited them given the opportunity) we got deeds of gift on the copies that we made and acknowledged them as any other gift. I preferred to have them made from the institution because we could control the type of processing and handling to a greater degree. We treated them as any other collection including issues of care and documentation.  We did not create a special category for them but noted that they were copies from originals on our cataloging records, donor records, etc.
 
 
 
 
 
 
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