Let's be quite specific about this, as it's quite easy to mislead by some comments.
B Stock is always purchased from the manufacturer by the dealer, in the same way that normal stock is. It has not been returned to a store by a customer and then resold as B Stock.
There are many definitions of B Stock, and they do differ depending on the manufacturer. They are usually minor blemishes at worst - anything worse is never resold back to dealers (although there are one or two places that do take this substandard stuff). It could be ex show stock, imperfect cabinets, faulty from new stock that has been repaired, products that have been loaned to dealers, and so I found out recently, stock that other dealers can't be bothered to sell off themselves (although that product was swiftly returned as it ws substandard). It can also be "old stock" where speaker manufacturers may have changed cabinet manufacturers and they have some old ones left.
Whatever the reason for it being B Stock, it performs exactly as a brand new, unused product would, and comes with the normal full manufacturer's warranty. It won't sound any different. I've bought B Stock myself over the years from a few different manufacturers and never had any issues. As already mentioned, it's hard to see why some products are B Stock. Blemishes are invisible to the untrained eye, and some speaker cabinets have a minute nick in them, which usually takes at least 5 minutes to locate.
The most common items are gloss finish speakers, especially black, as they have to be perfect to sell as new - it's very easy to mess up this sort of finish, so w see a lot of gloss black B Stock from many manufacturers. With speakers, it is only cabinet issues, and with electronics, it could be casing imperfections/damage - if any drive units or electronics have been damaged or are faulty, they've been replaced and fully tested to normal spec.
At the end of the day, it's a chance for those that want something a little better than their budget will allow, especially as we're all tightening our belts as the government squeezes more and more out of us.