Take great care in fulfilling your customers purchase/order.  Your customer will be able to tell if your priority lies in really caring about establishing and keeping your business reputable, or if you are in it just to make a sale.

Sure, making money is definately your goal, but you have to have the customers trust.

1. Answer your e-mails personally and with personality.  (Good personality.)  And, of course, in a timely manner.
2. Send an invoice if payment is not made immediatley.  Attach a friendly note and give them a reasonable timeline in which payment is to be made. 
3. Package your item with care.  If you are using a "used" box, make sure it is clean.  (Customers appreciate a packing slip with a friendly note.)
4. Ship the item as soon as possible.  There is nothing better than receiving your "find", sooner than expected.
5. Contact the customer when thier package is being shipped.  PayPal will notify them if you are using their shipping services.
6. Feel free to follow up in a friendly manner.
7. Your feedback stategy is up to you.  But be sure, when leaving + feedback, to include good personality.