Many items on Reverb are listed in "B-Stock" condition. These items may have been opened and then returned to the seller, may be a brand new item with a cosmetic flaw, or may have been used as a demo unit in a store. B-Stock items are guaranteed to function as they should, and may even come with a manufacturer's warranty.
It's always a good idea to reach out to the seller of these items directly to determine why they've been listed as B-Stock and what warranty options are available. You can message the seller of the item by clicking the "Message Seller" button on the item listing.
Feel free to check out this article for more information on the different gear conditions on the site.