Ive been wondering myself if there is graceful way of saying how much you appreciate feedback, a little blurb  that you could put somewhere on your listing where they will see it.

Totally pointless putting anything in the listing - it's not like the buyer will go back and look at the listing again after the delivery arrives.

The place for messages like you are thinking about is in the "thank you note" that goes inside the parcel.

The exact same place you need to write "please contact me at [email address] or eBay Messages if you are unhappy in any way"