That's good advice from Irene, but what you can do if you are looking to build up your feedback is hit a garage sale or weekend market. Grab a hand full of really cheap items (ones that have a chance at selling lol).
List them all at 0.99 with cheap shipping just to sell them off and get your feedback rolling. That's how I actually got my feedback rolling. For me it was sunglasses. I went to a garage sale and bought a box of sunglasses they had there for 10 bucks.
Think there was about a dozen pair in all, so I listed them each at 0.99 and shipped at cost. Can't remember if I actually made anything on them, but they all sold and I was away with good solid feedback.
As Irene said, definitely contact your customers with a thanks you for their purchase, and a friendly reminder that you would appreciate their feedback. Just make sure you have already left feedback for them first lol, or they may not be that keen on helping you out.