You would never... never bargain about foods and drinks be it at street vendors or at restaurants!!

It's alright to bargain at any street vendors selling clothes, souvenirs, bags, shoes, etc. For myself I use to say that at places behind glassdoors I wouldn't normally be bargaining.

But... when you are bargaining you have to remember that these people selling things on the street are hard working people standing in the sun all day trying to make an honest living to make money to pay their bills and their kid's school and education, etc. That means that you should treat them with respect - as they usually are doing to you. Thai people are friendly people so bargain with a smile... it's okay to joke! Be reasonable with your bargaining, and don't talk bad about the things they are trying to sell - then it's better to just leave the shop... still with a smile on your face, it costs nothing to be nice. And do remember, at the end it is the shop owner that sets the price, not you!

I say all this from experience because my Thai wife is running a street shop, and we are every day met by a few aggressive and unfriendly tourists who don't understand the real 'game' of bargaining. Have some fun shopping around!