You shouldn't really be "wary" of it, but you need to be aware. I purchased a 7D from them about a year and a half and it was a miserable experience. They claimed to have been "Scammed" at the time, insisted that I never payed for the camera (Although I had the receipt) said I needed to cancel the order and place it again, which I did, and still never had the camera in my hands. There I was, double charged, with no merchandise. They said I never made the first payment and so they could not refund it.
Eventually I had to file a report with paypal, and paypal was able to get a manager from Bigvalueinc on the phone. That is when they explained they were being scammed, placed all orders on hold (Understandable, but still frustrating) and after about a month of fighting, I got my camera shipped and the original $1700 refunded. Total nightmare. If the deal is good enough, like the current 5dIII deal is, I think a little hassle may be worth it though. You are covered by ebay and paypal both, just be ready to file that claim =)