B-Stock refers to items that were opened and returned, have a cosmetic blemish or imperfection (like a ding in the tolex of an amp or a scratch on the face of an effects pedal), or were used as demo units in a shop. These products are tested to manufacturer's specs and are guaranteed to function perfectly. Many B-Stock items come with a limited manufacturer’s warranty and ship direct from a brand or distributor who sells on Reverb.
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.