Buyer is requesting to return the item and refund because its illegal transaction. please help

[starminv]

I sold a second hand camera for a bit more than $1000, buyer paid on the same day, i have shipped the item by registered post.

Now the buyer is saying his son bought this camera and used his credit card paid without his permission. and his son bought few other things on ebay from other sellers without his permission aswell.

The buyer wants to return the item and i have refused him,

because theres nothing wrong with my listing and the item, i have stated "no return accepted" in the description, and this transaction has been completed already

the buyer requested to return the item for a full refund, because his argument is that this transaction is illegal, his credit card and ebay account has stolen and misused by his son.

is he entitle to get return the item for a refund?

am i obliged to accept his request?

Please help, thanks