This script will use what's currently on your shop manager page to point out items you are selling which are above market price, by highlighting your price in red. As it can only see what's on the page, changes which happen before you reload aren't considered, so keep that in mind. It's also programmed to ignore silver stars and magic truck vehicles at the moment (due to personal preferences), but there's a comment in there that's easy to find which will tell you how to add/remove items for that check.
To use the script, go to your shop management page and click the red link which says "[Check prices!]" in red at the top next to the link for viewing sales. If it finds something, it'll bring up an alert box that says some items are above market price. If it doesn't, it'll bring up an alert box that says all clear.
EDIT: Updated since original post to fix a problem that would occur due to only 1 comma being removed from numbers.http://www.soulraver.net/market_alert.user.js