First off, listen to DSM, it's solid advice.
Second. If you're really in a bind and must do this there is a right way and a wrong way. Do it the right way.
Ahead of time make them give you there doctors rec info,
ALWAYS verify it ahead of time
before making a delivery. When you are delivering it, first enter their house (leaving the product in your car), ask to look at their doctors recommendation to verify the info/name is the same they gave to you over the phone. Then make them sign a collective agreement (copy another collective's agreement, change the name to whatever you want to call your collective). Only at that point do you bring any product into their home.
That is not a guarantee that nothing bad will happen, but it should keep you reasonably well protected from law enforcement and depending on your county you probably are safe from the law. (that still isn't 100% legal, but it would be very difficult for law enforcement to mess with you).
Cops do bust people off craigslist, but using that method I've never heard of anyone getting busted. Usually what they do to bust you, is give you recommendation info on the phone, but when you come by for the delivery they ask to see a sample before showing you a doctors recommendation. Once you give it to them, then they bust you. If you make them show you their doctor's rec first and then make them sign a collective agreement before producing any products, they don't really have a case to bust you at that point.
This link should give you a sample collective agreement.
http://www.google.com/url?sa=t&source=web&cd=1&sqi=2&ved=0CCYQFjAA&url=http%3A%2F%2Fwww.safeaccessnow.org%2Fdownloads%2FSampleCollectiveAgreement-New.doc&rct=j&q=medical%20marijuana%20collective%20agreement&ei=lUcmTtvcCo-isAP87ZmcDw&usg=AFQjCNEzTlksZ5UYpeXUjVvFBGUQzxJV5g
If you do it the right way, you shouldn't have to worry too much about the law enforcement if you're in most places. (not promising you anything, just trying to help)
But as others have stated you're better off trying to learn more about growing top shelf bud then selling directly to dispensaries. The method I've described isn't necessarily legal, it's just what to do if you're in a bind for cash and looking for a way that will make it very difficult for law enforcement to mess with you.
Really what you should do if you want to do deliveries is first consult a local mmj lawyer for advisement, then form a mutual benefit non-profit corporation, start up a webpage, go to a local or state (not federal!) credit union and open a business account. Then you'll have a legitimate delivery service. Craigslist if pretty sketchy IMO. I'd advertise a webpage on craigslist, but I wouldn't sell directly off there if it were me. Safety concerns....