Communication with potential and actual buyers. There is a built in messaging system on eBay. It's there for your protection. And to keep out spammers doing deals off site is frowned upon and against eBay rules. So use the message system that's built into eBay. Keeping your communication businesslike and appropriate is a top priority.
That's especially true if you're dealing with an unhappy buyer. Your first reaction may be to tell them what you really think. But my advice is to step away from the computer, compose yourself. And then when you go back to write a response, make it appropriate and businesslike. A positive response even to a bad situation is what I call good customer service. I try to be like Nordstrom and answer any questions that the buyer may have or to resolve or fix the problem that may arise.
Perhaps you sent the wrong Item by mistake, you can quickly resolve it and move on. Accepting returns gives confidence to the buyer. Most of them are unlikely to use it but it makes them feel like they can do business with you. Here's a time saving tip. Don't let one individual transaction bog you down with endless negotiations, resolve the problem and quickly move on your time is valuable. Don't waste it.
Another best practice is to check your messages daily or twice daily or even more or better yet, turn on your notifications on your mobile device on your eBay app. And you can be alerted wherever you are. be sensitive to your buyers concerns. You have no idea what they're going through. So being your nicest is what I call good communication. Getting them with kindness is always a good road to take when you're dealing with eBay communication.