Points to remember:
1) never do anything too hasty. If you have a problem, give it time to work itself out. I once had a shipment going surface mail to Spain and the gentleman at the other end, whose knowledge of English matched my own knowledge of Spanish (nil), was understandably worried after waiting for 8 weeks. So,
2) always communicate as well as you can after an auction. In the Spanish saga, which ended up resolving itself after 10 long weeks, I had to use Babelfish to try and calm the gent down, and it worked! When he eventually received the goods, he left me some very nice feedback (well, Babelfish said it was nice!).
3) Don't plan on leaving any negative feedback at all. Maybe, in a rare instance, you will have to do so. But really, there is almost no need. If someone is really out of line, report them to eBay first. If they make a habit of doing something unethical or otherwise failing to play the game, they won't last long.
4) If in doubt, you can block bidders. I sometimes contact bidders who have a very low feedback rating, and on one occasion I blocked the person from bidding because I felt, based on the small but erratic feedback rating already obtained, that there was trouble brewing. eBay allows you to do this at your discretion.
5) Be aware that people in faraway places (like my buyer in Spain) often don't speak the same language. The only auctions I have had which went wrong involved people from Hong Kong, Malaysia and Taiwan. I simply did not hear from them after the auction (or in one case, I heard once and that was it). I still did not leave negative feedback. What's the point? There was some explanation, no doubt, to do with distance and language and so on (mind you, I did block these people permanently from bidding on my auctions, another thing eBay allows you to do). Anyway, I encourage you not to leave any negative feedback.