I would give him every possible opportunity to respond. I would e-mail him several times, over the course of 10 days to give him ample time to respond. I would specifically request that, if he does not want to take the part back, he should provide a proper response as to why.

I would send him a PM via MercedesShop just in case his e-mail is not functioning.

Since you have his address, I would send him a letter requesting that he take the part back.

If he still fails to communicate after multiple requests for dialog, via multiple sources, then he needs to be exposed. We would not want such a person to continue posting on the forum if he has no sense of community.