Lots of good advice here, especially from plaidlesspez and TheDarkTwins.
One thing I can add is if you need feedback since you're a beginner seller, only leave feedback for the buyer once they have left it for you. I am finding that I am always leaving feedback for people and they aren't returning the favor despite my amazing service. Unless it is a low feedback or 0 feedback buyer, which could be one of those unpredictable guys that leave you a negative or neutral for a pointless reason, give it to them immediately upon payment.
But once you build up a good enough reputation, then just start leaving feedback as soon as the buyer pays.
What do you guys think of this advice?