I've had a bit of success with pet related products. People seem more willing to drop money on their pets than their kids (judging from my toy sales vs pet toy/food/accessory sales). Take the time to scour Amazon's every category. Maybe try listing a few items from each category/sub category to see what sells best for you. Yes, that's a ton of listings, but sadly, putting in those hours and getting a feel for what sells/doesn't is the best way.
As Dagreton mentions, aiming for $6-$10 profit per sale is ideal. But if you're actually making profit, you're ahead of the curve either way. For the time being, while you figure out what to sell for more money, maybe scale up a bit so you're selling more things for a $3 profit. I'd rather sell 2 things for $3 profit than 0 things for $6 profit.
EDIT: for spelling