Ultimately, you'll want to contact the seller of your item to get their preferred return address. You can do so from your purchases page here.
If you have not done so already, you'll want to file a refund request to notify your seller you'd like to return the item for a refund. You can also do that from your purchases page by clicking "More Order Actions", then clicking "Request Refund".
Using this process will allow your seller to provide you with a return shipping label, depending on who will be covering the return shipping cost. The party responsible for return shipping costs is to be determined by the buyer and the seller, and is usually based on the circumstances of the specific return.
You can follow this process for purchases made on Reverb.com and Reverb LP.