DO require insurance and delivery confirmation on all shipments for your own protection.
DO accept PayPal.
DO ship promptly/when you say you're going to. Especially important at holiday time!
DO describe each item fully, with measurements, as much info as you can give, and a ton of clear & well lit pictures.
DO save your money on eBay's picture hosting and use an external photo host like Inkfrog, Auctiva, etc.---bonus, great templates for free which make your auctions look much better.
DO package things carefully and remember that in many places it's snowy or otherwise wet weather right now--a plastic bag encasing your item inside the box can save a lot of drama for you and the buyer. When in doubt, ship in a box rather than an envelope.
DO state flat shipping costs to all locations where you will ship. (You can research this on USPS.com prior to listing the item, just weigh it on your bathroom scale and add appropriate weight for the packaging).
DO try to use the free shipping boxes provided by the post office--make sure you use Priority supplies for Priority Mail, Express for Express Mail/EMS, etc.
DO print your postage online, through PayPal if at all possible--it's SO easy, and you won't have to stand in line to ship them. Your mail carrier can just pick up the packages from you in most cases. Awesome!
Lastly and possibly most importantly, DO have fun!!!