You should always be suspecious of stores/shops with goods priced in other currency than the domestic currency of the shop - most likely there will be some kind of currency conversion at some stage and you will loose on it for sure. Why would you even consider buying stuff in the airport? - way too expensive compared to what you pay elsewhere.

Anyway, most likely the stores will only be able to charge your credit card in Liras (not USD or EUR) so they need to convert to Lira (the exchange rate used is obviously at their discretion...)