Once you sell something on eBay and your customer paid for it, whatever you do, don't sit on the item for days on end before shipping.
Treat your eBay (or Craigslist) business like a business.
How would you like your items shipped?
Ship it right away.
Because if you don't, you'd be surprised at how quick someone is to comment on poor service.
I'm happy to say, even though it's been a few years since I've sold stuff on eBay, my rating is 100%. Not that I haven't had a few hiccups, but those were all attributed to either a misunderstanding or a delay in shipping (with the shipper, not me), and I always corrected it.