Artists and craftspeople (who sell on eBay or other online venues) get only one opportunity to make a "good impression" with a new customer. Your painting (Art Card, ACEO or craft item) might look fabulous on screen but if you neglect extra care in packaging, there may not be any repeat sales and future business with that client.
First, be sure that your artwork/craft is well protected. Not just for the usual bumps and jars of normal mail transit, but against dampness, high humidity and water damage. Always wrap or enclose your work in water-proof plastic before inserting into bubble mailer, envelope or box.
Second, you're an artist? Be creative! Write "THANK YOU" in bright colors on a scrap of paper - that's a basic minimum. If time allows, enclose a personal handwritten note, expressing your gratitude to your customer/buyer. In the case of a more expensive item, why not include a free bonus print or greeting card, made from your original artwork?
Third, if possible, wrap the item in attractive paper or colorful tissue paper (after the water-proof plastic) Make opening your package a special occasion - a treat - for your buyer. They will remember you, mark you as a favorite artist/seller and even recommend your listings to co-workers and friends.
Fourth, (this should have been point #1) identify your art/craft work. Label, mark, signature, tag, COA (certificate of authenticity) including your eBay ID, email address, website, blog, phone and/or any other information people need to get in contact with you. This is also crucial for posterity. Fifty years from now, will anyone be able to attribute this item to your handiwork? If it's important enough to have been made by you, then it's important enough to label it
Yes, these extra steps take a few minutes more but will yield great returns on your efforts!