Which rule sheet are you referring to? Link?
Each buyer will only purchase one pet from you. You may sell the pet on the black market or on the platform.
I would guess that the rule sheet may be referring to the last round where there are TWO buyers. The rules state that
The last round has 2 customers, and every player can sell one
pet to each. However, even in the last round, only one pet
can be sold from the platform. If you are selling two pets,
you have to sell at least one of them on the black market.
I have a hunch the rule sheet is referring to this case. I would have to read it to be sure.
(Love this game by the way... it's been too long since I played it.)