Can a seller call me a criminal in a feedback comment or accuse me of blackmail

As sellers can only leave posotive feedback and negative comments in posotive feedback are not allowed you can report it.

BTW  as you are also a seller add this person to your "Blocked Bidder List" in case they decide to act silly.

http://pages.ebay.ca/help/sell/manage_bidders_ov.html

I see it was a comment added to feedback that you left.

This may not qualify for removal I am not sure.

However add them to your BBL any way.

Sellers may respond to feedback however they wish, as long as they don't violate Ebay's 'inappropriate comments' policy. Had they left this comment for you as feedback, it would be a violation, but they may REPLY to your feedback freely.

The neutrals you left, one right after another, are going to make many sellers suspicious that you don't want to return for a full refund, but are trying to extort partial refunds and leaving neutrals when you don't get them. That may make some sellers reluctant to sell to you - they may remove your bids.