Hi and welcome to the Synth Zone.
B stock is a product that has either been on the sales floor as a demo instrument or it may have been purchased by someone and brought back. B stock can have small minor surface scratches, dents, etc. or other anomalies or it may not have any of those but simply was a demo unit that was played (most likely extensively but then again maybe for just a short period) and the keys may have minor surfaces scratches (from people playing it). B stock has the same original Manufacturers Warranty that a factory sealed item does. You can take your chance and get a B stock item and it may well be perfectly acceptable to you and you'll save some money in the process. On the other hand you may find the B stock item a little more than you bargained for and end up returning it because of it having too many scratches, dents, or other anomalies. Btw, B stock items are guaranteed that every feature on the Keyboard, etc., is in working order. That may not be much of a consolation though if there is a big dent staring you in the face when you play it. Some people could care less about scratches or dents as long as it works properly otherwise. Then again, some people won't tolerate any anomalies - me included.
Best regards,
Mike