When you sarch for "private feedback" in eBay help, it will take you to a site where you can choose whether you want your feedback to be seen or not
If it's switched to private, it's not possible for you to sell items.
I've chosen to set my feedback to private as I simply don't won't to show publicly what I've bought and how much I paid .... however, as soon as I won an item, I'll ask the seller whether I should sent him a screenshot of my current feedback, no one ever has asked me for one, though