I have 3 - this one, my primary buying ID which I occasionally use to sell personal items and post on the boards with. Another is my primary selling account, which operates a store and I have occasionally posted on the boards with it, and the last is relatively superfluous to my general requirements. I've only logged into it a few times, and made one purchase with it because I didn't want the FB for that purchase on any other accounts. 

It is not necessary for me to have separate accounts for buying / selling, as such, but it is most definitely preferable due to the record a FB profile creates, in much the same way as I wouldn't frame and display a personal purchase history on the wall if I had a B&M store.