I find a nice message through eBay AFTER they've received their item works pretty well for me. I prefer to send the note through eBay rather than directly via eMail. This way, the buyer is already logged into eBay to view the message, so it's a few less clicks for them to leave the feedback.
I generally say something along the lines of:
"According to our shipping records, we show that you have recently received the item you purchased from us. We just wanted to check in with you to make sure the item arrived safely and to your satisfaction. If so, we would greatly appreciate it if you could leave us positive feedback and 5 star Detailed Seller Ratings.
Both of these things are very important to the eBay community and maintaining a safe and fun trading environment for all members. If you'll check your feedback, you'll see that we have already left positive feedback for you.
If you are unsatisfied for any reason, or there is any reason you would not leave us 5 star ratings, please contact us right away so that we can make things right with you. We pride ourselves on our personalized customer service and we appreciate your business!"
I have about 3-4 variations of that statement that I send out so I can mix it up. That way if a person is a repeat buyer, the chances of them getting the "same" note is reduced, and just in case eBay ever decides to check messages, they won't see the same note being used over and over again on multiple accounts.
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