It takes some creativity to circumvent that, and a small (<$100) investment.
1. Build a small identity around the service. A website, a logo, and a few contact info.
2. Get in touch with a plastic card printing service, and have them get you one of these:
Note that they usually sell in pack of a minimum order. If you're lucky you might get away with ten, and cheap.
This is the final product that you're marketing and selling on e-bay. A small prepaid card, in the form of a code, in a PDF file.
3. Now it's up to you how you manage the rig.
Basically you can say "For TWFA(r) Co. Inc. $6 voucher card ", sell on ebay for $5, and your service costs $6 on the website.
On your website you have 10 more adjacent services (facebook likes, soundcloud comments, yt views, etc.) and you also sell those, so the card can be used for any of those services. Except those adjacent services are "out of stock". Or you can outsource if you find monkeys.
Aaaand that's how you sell twitter followers on eBay.