You call one of them - he claim he has the rights. You call the other - he claims he has the rights. Now what?

And try the other perspective... try to imagine that you are the store owner and now someone makes you responsible for making sure that none of thousands of your suppliers is doing some dirty business... it's going to cost you a lot of money and despite that you can never be certain and you will basically live in constant fear of a lawsuit. That's huge burden on any business and it's very inefficient. That's one of the reasons that we have courts... they have tools that business owners simply do not have and only they can (somehow) reliably determine who is doing legitimate business and who is not.