Most negative feedback comes from Miscommuncation between the buyer and seller. If you are a seller email your buyer congratulating them on the win of your item. This email should include payment instructions paypal, bidpay etc. or where to send their check or money order, This email should also include their delivery options (UPS Ground, UPS 3 Day Select, USPS Parcel Post etc.) and if applicable telling the buyer that you will email them with a tracking number when you ship. When you do this it makes the buyer feel as if he/she is your priority and that you work quickly. I can't stand it when I dont hear from a seller after days.
I try to keep a friendly realtionship with my buyers emailing them after they have received their item to make sure everything went ok and providing a link where they can post feedback.
Also I find it beneficial to place a note inside your package thanking them for their buisness and letting them know to email you first if their are any problems or concerns.