It's only a good idea if...
EVERYBODY uses it.
I got kinda perturbed when I saw a gut griping about how nobody used it...I had done a deal with him and left a positive post in the GBU (pre-karma), and he didn't (at that time..he did later) do likewise to me since I was a "buyer". The reasoning was it's hard to be a crooked buyer.
Yet I've seen several threads addressing deadbeat buyers and thought it was just as important for it to be a two-way street.
I have 8 positive karmas out of several dozen deals...about half of the karmas are for stuff I've given away, and I've given away much, much more stuff than that. I can't even give stuff away (and I always pay the shipping) and get karma.
As it is now I won't leave karma unless the other person specifically asks me to do so. And I will not ask for it.
So I just go buy gut feeling...I've seen JohnVH around long enough to know he's probably not gonna trash his reputation on the boards by ripping me off. Other posters I try to do a little search to see what their MO is...that's about it.
Karma and $1 will get you a cup of coffee around here.