When I flew to Florida for Christmas, I took a taxi from Netsel to the Marmaris bus station for about 10 YTL. The bus from Marmaris to Dalaman leaves 3 hours before any scheduled Turkish Airlines flight. The bus is called "Havas" which means "Air" (as in Havas Limani = Air Port) and costs only 23 YTL each way for the 1-1/2 hour bus ride. It is air conditioned and comfortable, with luggage stored below
Many cruisers advertise for taxi shares on the morning Marmaris bay net on CH 69 at 0900 to share the cost of a dedicated taxi to or from the airports.
My flight options out of Istanbul were not good timing, so I decided to spend a whole day exploring Istanbul. I stayed two nights in Istanbul at the Side hotel. This strategy, I highly recommend. My flight out the next day was at a comfortable 1145 am.
On my return, I collected my bags at Dalaman and walked straight out to the bus which departed immediately. Do not waste any time when collecting your luggage or you will miss the bus.