I don't see much of a conflict regarding the stamp. The store may not have had it made, but they may have been the ones to stamp the books. They could have gotten the stamp from the parents. If the son (whose given name may not have been Tom) was in the Pacific Theater, he wouldn't have needed his signature stamp.
I do doubt Bob's ability to recognize unstamped Reilly books. If the books sold as quickly as he mentioned, most of them would have been in his or his partners' possession for a fairly short period of time. I have a good memory (generally), but I wouldn't be able to recognize an individual copy of a book that I sold even 15 or 20 years ago unless there was something very memorable about it, like a name written on the front cover or a weird mis-cut. The Gilboy markings that some of them bear are not enough by themselves; it's probable that some non-pedigree copies survived from that distributor.