Should my son be allowed to sell his things to his friends?

Hi everyone,
My son is 10 and he loves Pokemon cards very much. We spent quite some money for his Pokemon card collection. He often trades Pokemon cards with his friends or other kids, which I have no issue with. Today he told me that he sold some of his Pokemon cards for 100 dollars cash to his friend (The $100 is part of his friend’s allowance). The trade itself is a fair trade, because some of the cards he sold to his friend are expensive. However I just don’t know if I should let him do this. His friend told my son that he will keep this trade secret from his mom. Now what should I do? Should I let my son give the dollars back to his friend and get back his cards or should I just let them be or should I tell his friend’s mom about this? I would like to know what do you think.
Please excuse my English. As English is my second language, I’m still trying to improve it.
Thanks
Jane