There is a fairly reliable way to tell what prices are/aren't accurate. If ever in doubt, go to the cards page, and see if you see a "Buy for $999999.0" like this:
If you do, then the price probably isn't accurate, and you should maybe do some more checking before making any big commitments. What usually causes that, is that for some reason our price grabber is only grabbing the prices from E-bay and Amazon, which can really mess up the Fair Trade Value sometimes. And many times when this happens, not even their prices are accurate.
We are currently working on finding and fixing all these problems though, but it takes knowing which all cards/sets are affected to do so. So please do let us know via a Uservoice ticket if you find any of these issues, and we'll do our best to get them fixed as quick as we can!