Coinbase has talked about fraud a few times. They obviously have a strong interest in preventing fraud, since they would be left holding the bag, if a massive price spike were followed by a massive drop.

They reportedly prevent about 3% of orders from going through for that reason (and I seem to recall there was someone complaining about it here in a previous HN thread, as they'd run into the fraud filters and couldn't place an order as rapidly as they wanted).

They also limit less verified accounts to buying (I think) 1BTC per day, so someone doing it to manipulate the market (rather than simply by BTC fraudulently) would need to create hundreds or thousands of fraudulent accounts, back them up with a US bank account (or some other supported funding source), and then get past the fraud filters.

To raise the daily buying limit you have to verify a credit card, and perform other verification steps. I think it's more careful than my bank about security; at least on the surface.