If you're in LA, I can tell you which dealers NOT to go to. McKenna and Savage = BAD BAD BAD. I'd go for a CPO though. It's like a 6 year/100K warranty. It's supposed to be checked over already, but don't rely on that. Dealers are notorious for just glossing over the car (dealer will claim otherwise, but don't believe him). Best to bring your own mechanic or ask to take the car out to an independent mechanic you trust before you buy.
Hard to tell when you're getting ripped off. Obviously, go to kbb.org and look up the price the car should be at. Then check other online used car sites for prices of similar cars. Finally, ask around on BMW forums like this one. Another one to try is
www.e46fanatics.com for 99-current 3 series cars.