There are many legitimate reasons why you might not have heard back from the seller (vacation, illness, power outage etc.).

Wait a couple of weeks to see if he responds. Resend your email. Then leave negative feedback if it is not resolved.

Looking at your transactions there are only two purchases you have made in the past 30 days that you have not left feedback for. One of those sellers has already left you positive feedback. If it is that seller you need not worry about relatalitory feedback because they can only leave one feedback per transaction.

Even if not, I think you should leave negative feedback if this is not resolved. If there is a retalitory negative what is it going to say? That you didn't pay? You can leave a followup comment indicating that you have the paypal receipt. It is usually obvious when feedback is retalitory and most people are going to understand. On the other hand if you don't warn fellow ebay users you open it up for this to happen to more people. Wouldn't you have appreciated a warning.

As far as vilating Ebay's rules, Ebay has almost no rules when it comes to feedback. But what is the other guy going to do, complain to Ebay that your feedback relates to an item he sold you when he was breaking the rules about off-ebay transactions?