I purchased and recently installed the $109 arm rest from Armrestking. The ordering process went smoothly and delivery was prompt. The arm rest was as described in their web site
armrest-autoaccessories.com - SMART FORTWO 451 2008 armrest bracciolo fortwo
My observation of the arm rest was that it was as described on the web site, though a bit pricey. Installation went smoothly until I released the parking brake. As I installed the arm rest as far forward as I could, the rear portion of its base prevented the parking brake from fully going down. A quick hit with a cutter removed the piece, and the parking brake then worked fine.
The fit is tight, tight, tight for the arm rest and seat belt fasteners, but it works and is comfortable. Time will tell if the plastic base of the arm rest holds up to the jostling it takes when fastening seat belts or "wide" passengers sit in the seat. However, it can be flipped back to make this easier.
Nonny