When my father in law passed, we found an envelope with over 500 $2 bills, he was a hard core collector…LOL
For World banknotes, I started out buying Euros & then as cheap as I could find dealer stock (which I don't recommend). I then turned my attention towards QEII nations: mostly Caribbean, NZ, Aussie, Falklands & Fiji (often in pairs or by bulk). I bought a few US notes but quickly realized I was buying blindly & felt they were too dear for how common they were. After that phase, I wanted special serials (low # or radars) & replacements from any country. I visited the Philippines & bought a bunch of special #. I have tried to buy strictly UNC pre-Euro but some sellers passed off inferior AU (or toned notes) as UNC so I stopped & turned to TPG notes.