This is simit, a kind of chewy donut covered in sesame seeds. Taste similar to soft pretzel. In the hotels there's a simit buffet eaten with an assortment of meat and cheeses. Of course, the price there would be many times more than here on the streets. Simit is a very popular snack as it can be seen on every street in the city, if it is not sold in trolleys such as the one above then it is carried in trays specially adapted to be carried on the head or shoulders.
The youngest vendor I found in the vicinity of the spice bazaar. He is seling vine leaves stuffed with boiled and spiced barley (I think). Just like in Greece, the vine leaves have been marinated thus having a sourish and saltish taste which is perfectly comlemented by the spiced barley in the filling. This boy does not just sell the food but also the gadget that he uses to make it.