You can keep it for transaction communication, if needs be, but NOT to send them advertising or promotional materials. That is considered spamming and can be reported, as eBay has a way to store sellers as favorites and the buyer can sign up to see your items by following you.

eBay tries to block the giving out of personal email addresses through the eBay messages system, to protect the buyer from unsolicited emails, and prevent off-eBay trades...not saying that is what you mean, but just their reasoning 🙂