Yes is yes, no?

Sometimes when I am asking a simple "yes/no" question of a Thai person who clearly speaks English, or I am asking a question in Thai to a person who clearly speaks Thai, they answer with "yes" or "chai" even though I am positive that they mean to say "no". I am often given a nod of agreement when I am certain they should be shaking their head. This took me time to understand, and I ended up with a few unfortanate purchases in or overly confusing taxi rides due to this miscommunication. I wonder if Thai people are taught that to agree is easier and more acceptable than to outwardly disagree or say "no". I must continue to get used to this.