This is the message on the final confirmation page on Turkish Airlines web site--before you purchase:

"Fares are only valid for sale in the country where the journey begins . In case the ticket collection is outside the departing country , fares will be calculated in the currency of the country where the ticketing is being made."

Yes I have purchased domestic tickets on TA while in the US. They just converted the cost in TL to USD using the currency conversion rate on the day I made my reservation. That is what the confirmation says: "fares will be calculated in the currency of the country where the ticketing is being made."

It does not cost you hundreds of dollars. If you credit/debit card charges a currency conversion fee you might have to pay that, but you would have to pay some currency conversion charge someplace anyway.

It is odd how often TA has better prices than Onur and Pegasus. We noticed that when we spent the winter in Turkey.