I am not sure if you are responsible for what you make or not, since they are the customer. It seems like if they "sell it", then the problem is theirs, not yours. Not sure.
The simple way would be to have some kind of standard "customer approval form" that needs a signature. Include in it a statement that the customer agrees that they own the rights to have their item produced by you, and take all responsibility for obtaining those rights, as needed. It does not need to be onerous, just something to cover you.
Just bury the text in a larger sign off document / order form / payment agreement that the customer signs.