You might find my blog post helpful.
Bulk Email vs. Spam
Bottom line: Your recipient ultimately determines what is spam and what isn't. And it's your reputation that is ultimately at stake.
The most effective email marketing establishes long term relationships over time. I suggest that you include an opt-in link in your autoresponse messages which are sent when somebody purchases your products and also using a subscription form on your eBay page where users can opt-in to your newsletter.
All the best
Tom