I bought the same armrest from that seller a few weeks ago and had no problems at all. I found one for about £28 though so its worth searching around. Its good quality for the price, although the top part isn't real leather (as stated). Fitting took minutes, with only 4 holes to be drilled and screw it down. I added 4 washers to help with stability. I love mine and feel the Jazz isn't complete without an armrest, I just need to fit my leather gear gaiter now to compliment it.
Regarding negative feedback, 14 negative in a month for a seller with such high product turnaround is nothing to be concerned about. They ship all around the world and sell very cheaply produced items, but there are so many idiot ebay'ers who will give negative feedback if their item is more than 1 day overdue or if their £2 item isn't of the very highest quality.
I've been an ebay'er for over 10 years so I know what buyers are like on there.