Haha, so you're the one that bought the ac-1 amulet from the start map shop.
A good way to not get screwed is to ASK OTHER PEOPLE. I mean it's quite obvious when you see a ac+1 amulet selling for 300 something, and an ac-1 amulet selling for 100 something, that the ac-1 is BAD.
It would be weird if AC-1 was good, as negative is bad. Just think of it as all positives have positive effects, and you're fine.