Whether you are allowed to use a purchased beat depends on which rights of use §31 UrhG you have acquired. The rights of use are defined in licenses. A music license is an agreement between the author and the user of a composition. It regulates the permission to use the music in certain media and clarifies the specific conditions.
Exclusive or non-exclusive?
If you buy a beat exclusively, it is no longer available for sale to third parties.
If you buy a non-exclusive beat instead (also called "leasing"), it will still be available for sale to third parties afterwards.
In some stores only exclusive content is allowed, i.e. releases and beats that you either composed yourself OR for which you have the exclusive rights. This includes:
A temporary license or a non-exclusive license is NOT sufficient in these stores. So be sure to check your user agreement to see if you have the necessary rights to upload your release to these stores.
Attention: Even if you have produced your song/beat yourself, it is no longer considered exclusive as soon as you have used samples, natural sounds, loops or similar.