Can i wipe clean a ps3 for sale while keeping the downloaded games on it? Or will that leave me vulnerable to cc theft or other fraud.
You can physically remove the hard drive, I think It's just a single screw.
Make sure to de-activate your Playstation account (or, specifically, the account that purchased the games) on the PS3 before you sell it.
You can only activate your account (and ergo play your games) on two PS3 consoles at the same time, so if you want to re-download your games onto another PS3 that won't be a problem provided that you haven't activated your account on two other PS3's already.
CC theft probably isn't a threat, but if you're worried here's instructions on how to factory reset your PS3: http://www.tech-recipes.com/rx/2265/ps3_how_to_factory_hard_reset/
protip: The two activations system can be used by two people to effectively make all PS3 PSN games half-price. "Gamepooling" actually used to support 5 PS3s.