In I Love eBay, http://www.skippingdot.net/ mentions three eBay ground rules:

1. Never buy something retail/new when you can get it cheaper on eBay.

2. Never let anything lie around unused: if you don't need it, sell it.

3. Pounce on obvious deals you know you can make a profit on with eBay.

These are pretty good, IMHO.