For intercity travel, what type of facilities are on the coach/bus ? i.e. is it air conditioned ? Where can we buy tickets?  

The intercity buses in Turkey are usually of very high standard (Mercedes model coaches, and all air-conditioned), but if you travel on a smaller bus used for short distance trips, it might not have air-conditioning. Drinking water is always served on midibuses and large buses free of charge. Also an attendant will offer Lemon cologne or tissue for refreshment, it is very typical in Turkey. 

The largest bus companies like Ulusoy, Pamukkale, Varan and Kamil Koc are serving most of the routes. You can buy the tickets from the bus station or one of the bus companies offices in Marmaris, most of which are located in Tansas supermarket area.