Dave,
I buy from them regularily.
As far as I know they declare if it is an original or copied slide/negative if they know.
For all the slides/negatives I bought I never noticed another one of the same subject coming up for sale yet. So I think they buy collections and sell whatever is in them.
Sometimes you can get some for real "cheap", other times the prices take off to the sky quicker than you can hit the "bid" button.
Very best Regards from Switzerland,
Urias