buying the design doesn't transfer full rights to you. as the creator, he has full rights and permissions over how the media is used. selling it you, he's obviously fine with you owning it and using it. if he doesn't consent to you selling it and making a profit off it, he's able to take action against you.

a very notable case is one that happened in toronto a while ago - someone was contracted to design and create a series of sculptures for downtown toronto. he did it, the city loved it. for christmas, the city decorated his sculptures with lights and wreathes and the designer didn't like it at all, and as he had full rights over how the media was to be used, he took them to court stating they altered the appearance of his sculptures without his consent and he wants them restored to their intended appearance, and he won, because he didn't sign away the rights in the contract.

think of it this way. i bought a cd. it's mine. i own it. if i start renting halls and playing it on a PA system and charging admission i can be sued by the band if they find out.