Aloha electricdigits,
That's a great question, and one not easily answered. I have bought a few things on eBay and, knock on wood, have had outstanding results. In fact, the only problem I experienced was with a video card I purchased for my PowerMac G4. The video card apparently did not work, but it was actually due to a faulty power supply (which subsequently died) and not the video card. I received the card, from Hong Kong, extremely quickly, and was able to return it for not only the purchase price, but the original shipping charges. Your money, assuming you use Pay Pal, is also insured up to a certain amount, providing some buyer's protection.
On the Craig's list side of the equation, I would say take a lot more caution. At least with eBay, you can check the seller's bonafides - with Craig's list, you don't know anything about the potential seller. I attempted to sell my last PC on Craig's list, but only attracted the attention of a scammer. Maybe my price point was too high, or maybe noone wanted to buy my PC.
I, personally, would only buy on eBay, and from a power seller with at least 96% positive ratings. Insist on seeing pictures of the actual iPhone to determine the physical quality of the iPhone before your purchase. That being said, I use an Incase case for my iPhone, but have never used any type of screen protector and mine is still pristine. As of now, the price point may be a bit more than you are willing to pay, but wait until the launch of the 3G iPhone - the price of the 1st gen iPhones may begin to drop on eBay. I hope this helps you in your decision.
HawaiiMacAddict