re: Got cancelation letter from OMV on car I traded in. What do I do?

You are fine. I would have suggested calling number at omv and faxed bill of sale to them. But regardless, when they receive it, they will see it was a trade in (our dealership has a 'garbage bin' we put plates in, state comes get them when it fills, so usually month or so before they actually get access to them, but we have forms that keeps up with that).