TPCalc.com is an amazing resource for anyone wanting to play the market in GW2. Whether you're an established trader or just starting out this program will do all your maths for you to let you know how potentially profitable an item is. This handy little site is very straightforward to use and I don't know where I'd be without it.
Even if you're not too bothered on exactly how much profit you'll make TPCalc will also let you know the minimum amount you need to sell an item for to break even:
Using this example I know at a glance that if I buy a garnet pebble (or any item) for 30c then I need to sell it for at least 36c to not make a loss on it. This works the other way around as well, if you put down a price you want to sell at the calc will give you a maximum price to buy at to break even.
TPCalc is absolutely free too, though you'll see there's a little unobtrusive 'donate' button there. This tool is so useful that if I had more RL money I'd consider donating a little in thanks for how handy this site is.
And as an extra bonus; if you use an android phone there's also an app version of the calc! Unfortunately I don't own a smartphone so I can't review the app (which is also free) but I've heard good things about it.
While I realise this was less of a review and more me telling you how handy this tool is I want to get into the habit of reviewing sites and resources so I'm going to end this as I plan to end all future reviews, with a little gold, silver or copper coin rating!