Mouse-over Gear To See Stat Increase / Decrease
Right now you can see if the gear a vendor sells offers a stat increase. Mouse-over the gear being offered for sale and you will see two boxes appear. The one on the right is what you are wearing at the time. Look at the numbers in parenthesis inside the right box. Green indicates an increase; a number shows you how much. Red indicates a decrease, with a number as well. No parenthesis means that the vendor's gear is equal to what you are currently wearing.
But! There is currently no way to know what that piece of gear looks like on your character before you buy it. That's not good.