dont forget to respond to communications in a timely manner
o all of the above
and
o determine your selling policies (over 18, CONUS, paypal+4%, shipping included USPS with confirmation but insurance optional/extra/at-your-own-risk, all sales final)
o keep copies of all receipts, emails, etc. a log file, or a folder dated with transaction. a year or five down the road you might wonder where your foobar knife is, and then go, ahah! i sold it to "that guy"...
o in particular with items that have serial numbers/etc, take photos and log this stuff. if you allow returns, you want the SAME item back. ask Best Buy about those Xboxes filled with bricks
o esp for CONUS, always ship with confirmation; unless the buyer says they don't want the hassle (of a signature); anything expensive, or rare, insure it, even at your own cost, though often i leave that up to the buyer.
o tell your customer all the shipping details when you've shipped out, including tracking numbers/etc, if any and/or be ready to provide them fast, if the customer wants them (not all do).
o part of TIMELY transactions is to complete all business BEFORE you go on a trip, leave town, goto jail, whatever. leaving somebody hanging for weeks or months is not only rude, you'll get nailed on the forums for it
even if it's an emergency travel thing, you can find an hour at some point within a few days of that to log onto the forums and at least tell some details; definitely email any customers of what's going on, if possible.
this has bitten more than one reputable source in the butt in the past.
o for general sellers: don't sell what you don't have or can't deliver.
o for makers, this means that unless you communicate differently: have it on hand or make a date - and stick by it. if you say 3 months and 2 hours 19 minutes, the customer IS counting on that. repeated slips are frustrating as hell and the customer doesn't care why. don't keep pushing them off because of other more interesting work. better yet: never take their money until it's ready.
o corollary: never pay for custom work up front unless there's something outstanding like exotic materials to be ordered or tooling startup costs. a small deposit should be fine. rest on delivery.
o always answer your emails/PMs. don't leave them hanging. be chatty

you should always make time for this.
o package better than anything you've ever received. i'm a little enthusiastic on this part, but nobody has EVER lost something out of a box i packed, or had it broken, shifted, etc. better than a birthday present
o be open to new ideas, haggling, trades, offers, something unexpected. if you MUST have the $$$, say that up front, saves time and anguish.
o be enthusiastic. this is out chosen calling/hobby. we're all buddies.
Bladite