It's a stall in a shopping mall, not a shop.
It's best if you banish all images of western shops from your mind when you go there as the culture shock can be debilitating otherwise. Most stalls won't try to sell you what you want, they'll try to sell you what they want. They can also ignore you for quite some time if they're chatting with the stall next to them. If they are having difficulty with language, they deny everything, hoping you'll just go away. There are other things that can be wildly confusing and illogical, but I'll leave you to discover them yourself.
If you've never been to BKK before, I'd advise getting your MBW fairly near the beginning of your trip in case you have to go back again and again. Oh, and once you've bought something from a stall, look back and if realise you were just haggling over a 20c price difference like your life depended on it and you walked away thinking it wasn't worth it otherwise, then you know you're embroiled in their game.