For information about arriving in Izmir by plane and getting a bus to Selcuk, see my post under Hotel Bella Selcuk.
We also took buses from Selcuk to Bodrum and from Bodrum to Izmir. Key point: buy your ticket a day in advance, if possible. On long-distance trips, the seats are assigned when you buy your ticket, starting in the front of the bus. And these busses do fill up!
The Selcuk bus station (Otogar) is new and very nice. We were on a medium-size bus from Selcuk to Bodrum; it did not have a steward. It started with just a few passengers, but at the next large town if filled to capacity and even two people were not able to get on.
The Bodrum bus station is not so new, but it is convenient, in the tourist section of town. Again, buy your ticket as soon as you know what bus you want. The clerks in Bodrum may not speak English; be sure to request the bus by the 24-hour clock (i.e., 13 hours, not 1 pm). Next to the bus station is a farmer's market, both outside and inside. Fascinating.
We had a large first-class bus Bodrum to Izmir -- fabulous! New, built by Mercedes-Benz, comfortable seats, a driver, a ticket manager, and a steward. On the 3 1/2 hour trip, first hour the steward put on gloves and served us cups of water. The second hour he served us cups of ice cream. The third hour he filled up a cart with nice size packages of a number of varieties of cookies and crackers, of which you could take your choice. You also had your choice of coffee or tea or several other beverages. Really delightful!
Note: the Izmir otogar is a LONG way from the Izmir airport. No direct transport except cab --- and that is relatively expensive. However, an enterprizing gentleman who needed to get from the otogar to the airport, like us, worked out a deal with the driver of a dolmus that if there were 6 people who wanted to go to the airport, he would make a diversion and take us there. There were 6 of us and he took us (and his other passengers, some of whom were not pleased at the diversion) to the airport. But it was all very entertaining as we bounced along and worked out a conversation with one of the other passengers. Because the driver was not allowed to take the dolmus up to the entry of the terminal, he dropped us all off about a block from the international terminal (we needed the domestic terminal). There were lots of cabs who showed up willing to take us the rest of the way. The Turks are friendly and honest; being adventurous is fun!