Dick Greenhaus wrote:-
"As a retailer who tries to be both honest and ethical, Am I to he held personally responsible for the behaviour of my supplierss?"

Well, if you are really trying to be "honest and ethical", what do you think the answer to your question is?
If you know or suspect that your supplier operates in a dishonest way, are you happy to continue supplying that person's goods?

Dick Greenhaus wrote:-
"I frankly have no idea of how to properly recompense source singers"
Well, there are honest and ethical suppliers that try their very best to contact the artists who have recorded their traditional songs... or their descendants.

A singer finds out that their songs/music have been released or supplied by your business and they know nothing about it and they are unhappy about this. How does this make you feel?