Who knows how to specify point of origin in GDS for availability searches? Of this is possible we might be able to get this done in EF.
For example (and forget for a moment TK is not available at the moment), availability for IST-AMS on TK for same flights will be different for travel originating in IST ans AMS. Hence a query for IST-AMS will likely return wrong result if you are looking to change the return portion of your AMS-IST-AMS.