Bob Leduc has an nice article up on Home Biz Tools that explains why post card marketing is effective for all kinds of businesses (including eBay, and for driving traffic to sites/listing pages/eBay Stores).
I would just add a few points that will help you get your post cards read.
1) If the recipients know you by face, name, or company name include that prominently on the card. People like to see people and know people and hear from people. Leverage that, especially if you’re sending cards to people who hear from you on a regular basis (as with email newsletters, regular business communications and so forth).
2) Write a headline for the card that speaks to or is about the recipient (not about you or your company or your product). Grab their attention, but make the recipient the focus. For example, “How Often Do You See Limited Edition Widgets for Less than $100?” speaks to the recipient. And, “The Top 10 Causes of Dry Skin” makes the recipient’s problem or need the focus.
3) Put news in the title: “New Orthopedic Neck Brace Helps You Sleep More Soundly”
4) Make an offer in the headline. “Summer Sale Starts This Friday — Free Shipping for Valued Customers Like You.”
There are plenty of other strategies. I’ll cover more of these in future posts. Remember, you can mix these up, too, and create layered messages. That includes mixing the photo concept with the headline.