The answer to the question above is very simple. The reason why you should list your items in Marketplace Second Life is because of the fact that hundreds, if not thousands, of people visit and buy items from it.
Listing your items in Marketplace Second Life is FREE but Linden Lab do take a small commission from each sale made in the Marketplace. Still, the commission is a small price to pay for the convenience provided by the Marketplace to all Second Life members, buyers and sellers alike.
My blog, http://Kabalyero.info, has the full story (http://www.kabalyero.info/2013/04/marketplace-second-life-and-linden.html).