When I tried to list my 2G Iphone, the initial listing was removed by Ebay and, a very polite email was sent to me explaining that my photos of my Iphone needed to include my Ebay user ID in them. (Just a piece of paper with it visible in the photo was fine.)
That process cost me a day or 2, and not all that tough to fix, but unexpected.
I've also experienced Ebay/Paypal scams (usually originating in Africa) when selling other items, so yes, be careful.
If someone sends you an email saying they paid, don't click on their links. Start up a browser and check from scratch.
When replacing my old 2G Iphone on one occasion I kept my SIM card, thinking it was the only way to keep my phone number. However, when I upgraded to a 3G, the people in the Apple store said I didn't need to keep the old SIM card and just did something on their end to associate my old number with my new phone.