I didn't see this mentioned before other than on one or two tripadvisor reviews after I returned, so thought it would be helpful if everyone knew about this sneaky currency switch they have going on at Ataturk airport's duty free shops. I learned this the hard way as I was rushing to make my purchase after my flight had already started boarding.

Everything is priced in euros, but if you try to use your credit card, they will run the transaction in liras using an unfavorable exchange rate. The amount they charge in Liras is 4% higher than what you would be paying using the usual visa/mc conversion rate. You will not be given a choice of currencies and you won't know what they are charging in liras until they hand you the receipt. And it is not possible to know you're being charged for the currency conversion unless you take out a calculator and try to convert the liras back into euros.

To avoid this, make your purchase using euro cash. If you don't have enough euros in cash, you can exchange it at the bank across the hallway before making your purchase. ....or mentally add 4% to all the prices you see there before deciding whether to buy.