TEchnically you're still buying from Origin, but that's beside the point. I have bought both Origin and Steam games from a number of sites. My two cents are:
Of course, look around for a deal.
And of course, read reviews of the site before using it if it's not a well known one (Amazon, whathaveyou) AND more importantly, before placing the order, check their payment options (make sure they offer known secure methods).
I tend to always go with Paypal, it is easy, and Paypal offers an extra level of security because if you contest the purchase they might refund you even if the store does not if they deem the store being fraudulent.