I was there this past summer as well as five years ago. I think you should keep your expectations very low. I have never seen any serious coin dealer in Turkey (Antalya, Istanbul, etc.). For one thing, if you're looking for ancient coins, it's a criminal offence to sell them, so no-one deals with them. As for more recent coins, I have seen no dealer in those either. You may see a few Ottoman coins in antiques shops, but you'll be lucky if they're authentic. As for actually buying them, there won't be a price tag, so you better know your coin values otherwise you'll pay five times their real worth.
My advice, then, is to not even bother. If you see a shop and decide to go in, have a look at what they have on offer but don't buy right away (though they won't let you go easily). Go back to your hotel or a restaurant and do a Google search...
