Bargaining!
Foreigners may find price haggling ludicrous, but it is nonetheless common in Turkey. The usual rule is that if a price is displayed, the vendor is unwilling to negotiate. This includes sophisticated retail malls with barcoded price tickets, pubs, restaurants, and minor purchases from the local market, such as half a kilogram of apples. Haggle over large purchases such as gold, leather, and Turkish carpets.
Always double-check pricing, both on menus and on the bill or taxi meter, before paying, and question any inconsistencies. In carpet shops, some haggling is common, so study the price and currency they're quoting first. It's completely OK to negotiate a lower price.
Whether it doesn't work, stand up and declare you're out of time before double-checking if that's the most they can do. The most important thing is to purchase something you enjoy.