Here’s my template, tweak it to fit your needs.
Dear (write in the buyer’s ID or name),
Thank you for your eBay purchase! Once you receive your item, please remember to leave positive feedback! I will do the same for you in return.
Also, make sure to bookmark (Your Seller ID) as a favorite seller.
Thank you again, it has been a pleasure doing business with you. If you have any questions, please contact me directly at (your email address).
(Your Seller ID)
I paste the above into a one-page Word document (3 or 4 will fit on one sheet depending on how you set up the margins). Cut them and use for each item you sell.
This enclosure is not only a nice gesture, but it also reminds the buyer to leave feedback about their purchasing experience. Also, if for some reason they had an issue with the transaction, they know to contact you directly instead of leaving neutral or negative feedback.