How to trace specific ownership marks and marginalia? How to go about reassembling a long dispersed library? A day of testing several sites with provenance research options left me with ambiguous feelings. It appears that searching online for specific traces of use is not only a time consuming but also a daunting occupation. Unsatisfactory, because provenance records are not always included in descriptions and even then they are not always mentioned in the online entries. I entered the name of the well known bibliophile Marcus Fugger in the C.E.R.L. Thesaurus and got zero result. I find that hard to believe. The thesaurus is linked to a long list of European libraries and zero result would mean none of these libaries have a book formerly owned by Marcus Fugger? Ideally, every book should be checked from front to back for traces of use and a thorough description made of the evidence found, according to standard, searchable entries. A lot of work and not enough librarians to do the job. The ambiguity lies therein that looking for traces of use is a mountain of work and this is a good thing because that mountain covers a wealth of unidentified gems for the lucky finder.  
To end on a positive note, the provenance page of the library of Lyon is a feast for the eye. Multi searchable, very complete descriptions and with photographs of relevant details.