The sourcing of artefacts of any nature is a painstaking process. A central database of "Who owns what" does not exist nor a record of recent trade or auction. Private sales are not recorded and museums while cataloging will not make that the material public.
The market is awash with gossip of who bought what or selling something. The experienced man collects these data and files them away and correlates the tidbits of information till he is a picture and will then act in a circumspect way as not to alieniate potential sources as the collectors are all jeleaous and conduct secretively.