A range of duty-free is available when you arrive in Turkey before passing through customs.
The following can be imported into Turkey without incurring customs duty: 200 cigarettes and 50 cigars or 200g of tobacco and 200 cigarette papers or 200g of pipe tobacco; 5 bottles (1l) or 7 bottles (700ml) of wine and/or spirits; reasonable amounts of coffee and tea; 5 bottles (up to 120ml each) of perfume; gifts up to a value of €255.65 (or equivalent); electronic goods up to a value of €255.65 (or equivalent).
An extra 400 cigarettes, 100 cigars and 500g of pipe tobacco can be imported if bought on arrival at a duty-free shop.
When you arrive at customs details of any large, valuable items like electronic goods, or your car if you're driving, will be entered in your passport. These will be checked when you leave as it is forbidden to sell them during your stay.
Turkey has strict rules about exporting antiquities, with severe penalties, including prison sentences, for anyone caught smuggling them out of the country. If you are in doubt about whether something you want to buy is an antique, check with the manager of a local museum.