Why are there so many used B&W speakers for sale.

+1 @petg60 

"First excitement, then fatigue."

This supports my argument that we each need two systems:

A hot, exciting system that you turn on after coming home from work. A system that "gets you going" quickly for maybe an hour or two.

AND 

A more laidback system for the week-end that you listen to for hours-on-end.

Initially they sound great but can wear on you over time, at least in my experience YMMV

First excitement, then fatigue.

+2 @erik_squires 

Mass market loudspeakers that people enjoy and many upgrade by moving up the line.  

+1 @erik_squires 

I can think if very few audiophile speaker brands that have enjoyed the enormity of success for as long as B&W has.  When you sell thag many for as long as they have you are biund to have a big presence in the used market as well.

You have answered it yourself.