Siobhan, I've just paid in full to an owner who advertised on Owners Direct and I had similar concerns to you. OD do offer a basic rental guarantee to prevent you getting defrauded although based on my reading of the small print it's not exactly iron-clad - quite a lot of hoops to jump through etc.
On a more practical level, they suggest on the site that you call the owner before you pay to check they are a real person etc. I also had a sneaky peek at our owners on Facebook. At least that way you know who they are in case you get ripped off. Beyond that, I suppose there's a bit of risk in any private transaction but hopefully it will be fine...