Where to buy used car in Riyahd

There is large car market on Khurais Road in Riyadh.

Most car dealers are also on the same road.

You can check BMW dealer, Used cars branch (if you want a BMW) at Dhabab St.next to the chamber of commerce. You can get 2011 year models for 30-35% cheaper than brand new models.  Some car dealers have a guaranteed used cars section. Car dealers are mainly located at Khurais road as SaudiUK said.
There is two main used car markets in Riyadh, on ein Shifa and one at Khurais road exit 28.

check out Abdul Latif Jameel. They have used car branches in Khurais, Naseem, Olaya, Shifa and Badia. They are known for their quality. The prices are a little above the average but they offer warranty and also you can start a lease with them at zero downpayment.

Any preference for brand or type of vehicle you are looking for?

I know an expat who wants to sell his KIA CARNIVAL 2007 which he has used for about 4 years and done about 180K km's for SR 25,000.

Its a 7 seater, 2 in front and 3 and 3 in two levels at the rear, with rear sliding doors. Body has a few scrapes ad scratches which is qute normal in this neck of the sands.

Funnily, the engine is a 2009 which has done only about 100K km's since the KIA agents replaced the old 2007 one for him since they goofed up on it once when he had out it in for a full service 2 years after having purchased it brand new in Sep 2007. This is not normal in these parts but the Chief Engineer at the workshop was kind enough to offer this solution since he accepted their goof up willingly. Fine Lebanese guy whom I have met and know well, now, too. Give me a shout if interested and I can negotiate the sale, without any commissions, of course.

To save time you can try searching online. Abisayara.com has posted a lot of inventory from both brand distributors and smaller dealers in the major cities.