At the moment https://coins.co.th/
seems to be the easiest way to buy and sell Bitcoin with Thai Baht. ID verification is required after opening an account, i.e. send in a scan of the passport (or ID card for Thai citizens) and a scan of a bank or insurance statement or similar. And you need a mobile phone (can be both a foreign or a Thai mobile number) for receiving SMS messages with passwords.
Deposit cash at all major banks in Thailand. In most cases you can even deposit cash at CDMs (cash deposit machines) and some cash-accepting ATMs. Then go online, log in and buy Bitcoins.
If you want to sell Bitcoin there is currently only one option for foreigners to cash out: receive up to 19900 Baht per order at any Krungthai ATM. No credit card required. You receive two passwords independently, one to your email address, and a second one in an SMS message to your mobile phone.
EDIT: this option is currently deactivated.
Thai nationals can also cash out at 7-eleven (up to 5000 Baht), at most post offices, and at most branches of the major banks.
Fees for depositing cash is around 200 Baht. Withdrawal is 30 to 50 Baht. The exchange rate is the medium average rate plus/minus 0.75% (credit cards: 2% to 4%, W.U.: over 5%).
There is also bitcoin.co.th and bx.in.th, but their cash-out/cash-in options require a bank account.