Are you by chance referring to an Esoteric seller? The seller should be aware of what he is selling if he is the first seller.
I will add one more version of "B" stock which has become increasingly popular - show stock. If a manufacturer brings several items to a show to demo as well as pass out to certain folks to 'try' while at the show, when these are returned, they label them 'B' stock.
In addition, as new release are on the horizon, you will see manufacturers reduce prices to dealers (and ultimately to use here on the preowned market) giving them incentives for retail and employee purchases.
Have you ever noticed a certain item just DROPS on the preowned market? In these cases it is likely some type of employee incentive/discount (those that are buying at 30-40% of retail and looking to make 5% immediately even when they are supposed to be holding the item for a year. What this does now is change the supply. More items available, and at a lower price than the rest of the folks selling the same unit. It now forces those selling the same unit to compete, although not necessarily get lower than, the 'cheap sale'. Book values drop and thus the inevitable:
"One just sold for $5,500 will you drop your price $1,800 so I can buy yours?"
Don't be concerned with "B" stock unless it has surely been repaired. I personally am not a fan of repaired goods, unless it was a Mark Levinson or Krell item.