Be careful, if eBay are pulling your listings every month then you will eventually get banned. And it can be a nightmare getting your account activated again once the suspension period is over.
If you use eBay as your main selling point then offer postage on your codes, then as stated above, just tell the person if they need it quick you will just email it to them, 90% will accept this, the other few you will just have to stump up for postage.
Wait for people to ask for their postage costs to be refunded for digital delivery, you will be amazed at how many let you keep it if you don't mention it, meaning more profit for youself.
