My wife is breast feeding our son. However, we weren't always sure she would be able to do it. So before she gave birth we went to all the different formula makers and signed up for their mailing lists. . . and in roll the coupons, and better yet coupon checks.
Coupon checks are actual checks. Checks that are only good if you use them for formula, but checks none the less. This means you can use more than one as opposed to a coupon where you can only use one per order. The company can track that you used the check as opposed to a coupon where they don't know if you used it. So if you use it, presumably, they'll send you more.
Why did I just describe all that? Because these elements make for the perfect ebay item. Perfect for the seller because they can sell the checks for almost face value. Perfect for the buyer who just needs to buy them in bulk and use several in one purchase, they're making money. . . slowly admittedly, but making money. AND once the new consumer uses the checks the company sends more.
So far we've sold $31 worth of checks for $28!