Several banks have branches on the main street of old city, complete with ATMs on the exterior walls.
There is a big-box type store Migros in YeniÅehir, right at the beginning of the ascent towards the old city.
Stores in old city are closed by 9-10PM even those few that are offering alcoholic beverages, which are typically open till late at night in western Turkey, so make sure you have enough supplies of snacks and drinks especially water! for the night.
Colourful keffiyehs locally known as poÅi can be a good buy while in Mardin. There is a store on the main street of old city where you can get 4 scarves for 10 TL. The red, yellow, and green kuffiya is the traditional and politically loaded colors of the PKK/Kurdish, and will get you lots of friends in the Kurdish areas but don't wear it in the Turkish areas.