When a client invites you to work on a project, he may indicate if you're allowed to resell the final product in the marketplace. If you don't see such indication, then you must asume that you're not allowed to resell. It's also possible that the client forgot to indicate this, or he's simply not aware about such option, so it never hurts to ask him about that. In any case you must remember, that without explicit permision from the client, it is NOT alowed to resell the product in its original form, or any derivatives. I would advice in the future to clear these things with a client before you'll start working on the project.