I don't know how often people think about this, or if its happeed to anyone else, but theres a default option in the UI thats costed me probably 1500 L$ in accidental purchases. Fortunately most of them ended up being decent items, but it still bothers me that this how it is.
When you select "Buy" and leave the diologue up, the default option is yes, "BUY". Now... Suppose you take a last pause to say something? You hit enter to bring up the chatbox, and... oops. You just bought it. Even though the buy window itself wasn't the highlighted, active window. (You clicked in the world somewhere.)
So the suggestion is, to keep things simple, make the buy diologue default to "Cancel" instead.