Well, that was certainly convoluted! I get what your trying to say, but have no idea what answer is appropriate overall. I do know which answer is appropriate for me and that is that if A pays for return shipping A gets said item back. If A does not pay for return shipping A gets both diddly and squat (collectively) and B will likely ship the shirt to C (assuming C wants said item). Not sure what B will do if C does not want the item though. I like the art, but the quote, not so much.
Now this is one where lawyers would really be useful.
Person A is offering for sale object O, something they do not have the right to sell. Person B knows this but agrees to buy O for a sum of money M. B receives O and somehow still has M or regains M. What legal obligations does B now have?
If O had been a stolen object, then I suspect the law would be simple. B would be entitled to keep the money but would also be obliged to do their best to ensure that O was returned to its rightful owner (C) otherwise they would be guilty of a criminal offence of knowingly receiving a stolen object. B would have no obligations to A as the contract to sell was unlawful given that A had no legal title to O.
In this case things aren't so clear cut. The design belongs to C (Rich), the concept to D (the Beeb), the shirt material and ink to A. So what is B obliged to do when they find themselves in possession of both M and O? If they painted over the design and returned the shirt to A what would the legal position be? Could A sue for damage to the shirt when they have no right to the design on it? If B placed the shirt in C or D's hands and informed A where it was, what laws could A use against whom?