*EDIT: I almost forgot. You're working with Ebay, so make sure the sales letter doesn't actually sell a product itself, and don't make the website you direct them to look like it's related to your Ebay account *at all*. You really don't want Ebay to be able to slap you saying you're selling products outside of their service to avoid the fees.
First, if you don't already do this with your ebay biz, you should immediately come up with a physical sales letter, or business card to send along with any purchase you mail out, and develop an additional funnel to send those purchasers down. As for monetizing after the fact if you haven't done any follow up, add them to a list, and send an initial email something along these lines:
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RE: Your Ebay Purchase from (sellername)
Hey, this is _____, your friendly ebay seller! I just wanted to let you know that I have left positive feedback for your ebay account, and would appreciate if you would do the same. I hope you're enjoying the (product they bought from you). If you have any concerns, email me back and I'll see what I can do for you!
Also, I know you're interested in ____, and I thought you should know there is a great opportunity at _______ (fill in sales letter here).
Sorry if you aren't interested, but it's a really good opportunity! If you're not interested in receiving offers like this in the future, simply click here (unsubscribe link).
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If they don't unsubscribe within a week, move the email over to your primary list. I'm about to do something similar with an email list of 30,000 or so customers(website, not ebay). I recently discovered one of my SEO clients doesn't monetize their customer list *at all*, so I got the owner to agree to a JV with me utilizing his list and products. Wish me luck!
Make the email as specific as possible, based on the products that you're selling and your form email capabilities. You should be pretty in the clear, as this initial email is primarily a customer service letter. If you're super worried about ebay slaps, then instead of selling something in this letter, adding them to your list and giving them an unsubscribe link, you would want to give them great information and offer them a subscribe link (You'll end up with a lot less conversions this way though).
* - A third option if you're worried about ebay slaps and not concerned about developing a long term list is to just send a single email with the sales offer, AND a subscribe link. (If you aren't interested in this offer, click here to receive future offers), and dump the emails after.
If you want to share your niche info on this project, send me a PM and I'll help out with the sales letter. I'm not a sales copy expert, but I'd like to think I'm ok at it.