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.