Spot on, they will let you know if something is not right.
Feedback is voluntary and many just don't bother.
Some buyers won't because the seller is asking for feedback to be left.
As long as all your feedback received is positive, the rest doesn't matter.
Good accurate listings, fast postage and packed well, should mean trouble free transactions.
Same, I have 2354 feedback for around 5,000 transactions.
Quite agree, many buyers don't even know to leave feedback.
@k1ooo-slr-sales wrote:don’t send messages to buyers about feedback, eBay will do that.
Good point, eBay send a reminder to leave feedback I think it is around 30 days after a transaction.
I have this thing where if someone asks me to do something I was planning on doing anyway, suddenly I begrudge them and don't wanna do that thing at all. 
The best thing you can do - which many be easier said than done - is forget about feedback. Focus on the things that you can do to provide the best possible service / experience for your buyers, and the rest will follow sooner or later; though I suspect you're already ahead of me on that if you're getting private messages or cards in the mail. People who take the time to do something like that will also probably sing your praises elsewhere if the opportunity arises - that's worth significantly more than eBay feedback, IMO. 🙂
