I'll bet the B stands for Black. So to answer your question, no difference between 686 and a 686B other than the B is black finish.
I have seen demos/open box 686's go for around $420 pair at Magnolias. These would be with full warranty so if I was in the market for a pair of these, I would inclined to go this route instead of $350 for used as you were quoted. Unless the $350 had a warranty. Then that is a good price if they are in very good condition.
I have never heard 686's so cannot comment on how they would sound compared to other bookshelf models in this price range. I have only heard the 683's and found them to be very engaging and strike me as a lot of speaker for the money. Definitely the star of the line up as was poined out by jima4.
the $350.00 used to be authorized dealer, but lost it when magnolia best buy stores started selling b&w. the store seems to think that b&w would still honor the warranty but for some reason I don't think they would. what happened was b&w told a lot of dealers if you can't sell as much as best buy magnolia you are no longer authorized. they couldn't do it. that owner told me he knew a lot of other rotel and b&w dealers who also lost the authorized status because they couldn't sell as much.
The "b" indeed indicates the black ash finish for the speakers. The 686s are great for surrounds or in a very small listening space. As jima4a already mentioned, the 685 and 683 are the best speakers in the line.