Before accepting any argument as true, even -- or especially -- if my initial inclination is to believe it, my first reaction is always to consider if the opposite argument is tenable. One way of doing this is to think about what sort of evidence would be required to either (i) disprove the argument, or (ii) prove the opposite argument, and then look to see if either sort of evidence exists. 
In the present case [discussed in the preceding two blogposts], if we were to take as our working hypothesis that the Lomax-Wade collection consisted exclusively and entirely of cuttings bought by Webster at the Celotti sale (rather than acquisitions from elsewhere, such as the Ottley sale), I can envisage three main ways of disproving this hypothesis.
One is to find a Celotti-Webster item that was definitely not in the Lomax-Wade collection, but proving a negative is often impossible, and in this case there are too many ambiguous descriptions in both catalogues to state with certainty that an item in former was not in the latter. A second approach would be to identify one or more items in the Lomax-Wade collections that were definitely not in the Celotti sale; but this would be very difficult to prove for similar reasons. A third, much easier approach, which avoids the problem that we do not know the subjects depicted in all the initials in the Celotti sale, is to see whether the number of items bought by Webster at the Celotti sale matches the number in the Lomax-Wade collection.