Hmm, according to rms buckler does work for merchant classes. In fact, maybe I'm missing something but the only classes I notice that can't use buckler is hunter, mage classes, and extended classes. Guess it's up to you to decide whether you want to be able to use it on any of those classes or not, but you could no doubt always sell it later and buy a different one if you needed. I personally would get the buckler since I'm only likely to use it on my Blacksmith anyways, although since it's only really a difference of 1 def, it might not matter that much.