If you are in an industry where you are constantly meeting people, introducing yourself and wearing those little "hello my name is" stickers, perhaps you would like that on your cards, here are some design options:
Do you consider yourself a hipster? Do you love psychedelic designs of fractal artwork with lots of colors and movement. Define your hippy attitude with a set of custom personalized business cards. Here are some groovy designs:
Get more feedback after your auctions and return customers by including your contact information when shipping eBay items. Here are some great sets of designs for custom eBay Business Cards. If you are a high volumn power seller who runs their own store and sells items that may have a lot of return customers and repeat business then you want to make sure they know how to find you again.
Offer a coupon code or discount for your auctions:
People don't bother to bookmark or save their auction history and if it's past the 60 or 90 days that eBay shows on their reports a person may not be able to remember who they purchased something from before. If you include a business card, it's likey they may save that and use it for future purchases. Plus if you have a website where you sell good direct and offer a slight discount, this is a way to cross promote that as well.
Create custom polka dot business cards. This fun pattern will never go out of style, it's been around for decades and induces a feeling of fun and whimsy. Great for many types of companies or if you just like a little bit of child like cuteness in your card design. Here are some of my favorite styles from Zazzle.com: