Travelling in Thailand by bus can be a great experience or a nightmare mainly depending from your luck. Some facts will be useful for you to know:

I. There are many bus companies in Thailand and not all, unfortunately, are reliable.

II. Car accidents are very frequent in Thailand. Even if you are not directly involved, the accident will sensibly delay your travel for hours.

III. Buses are quicker than train, but not necessary cheaper.

IV. Booking your bus through your hostel / hotel is your best bet to avoid a scam.

Here some of the most likely scams to happen (so for you to know them and possibly avoid them).

SCAM 1 - You have sold a ticket for a bus that would never turn up

SCAM 2 - The bus arrives but withy various excuses stops for a long time in front of shops / bars / restaurants where you will want to buy some overpriced food and water to survive.

SCAM 3 - The bus drop of you off far away from your destination forcing to take a tuk tuk overpriced lift to finish your journey.

How to deal with these scams? 

Best suggestion is to remain calm and not desist from arguing. 
As a last resource may work to say that you will later contact the police to denounce the scam.
(Tourist Police free phone number: 1155 with every kind of phone) 

You may also interested to read more info for travelling in Thailand by train, taxi / tuk tuk, scooter.

If you are planning to travel in Thailand visit our page about Thailand where you can find our list of posts about destinations in Thailand, our recommended itineraries and more practical info.