I don't remember reading anything about a buying limit in DiceMasters, so maybe now I'm getting confused with Quarriors (where I believe you could only buy 1 copy of a die at once even if you had 2 buys...).
So, is there any limitation when buying? specially regarding Basic Action Cards. I mean, if I have enough energy, I can buy the 3 dice (cost 3 each) on an BAC, at once, right?
Correct. The only limitations on spending energy are some Globals that limit their use to once per turn.