A seller isn't able to close their own account for a certain amount of time after their last transaction (I'm not sure if it's 60 or 90 days), and presuming the item is still showing as paid/posted in your purchase history (which suggests the the listings themselves weren't suspect and removed by eBay), I can only guess they've been de-registered for something like non-payment of fees, which can be just temporary and their account would be fully restored upon payment of fees (or other resolution to whatever is happening).
I'm not sure how much access they would have to eBay account functions, but unless there are other alarm bells, I would wait the average time and/or attempt to contact them at the email address the PayPal payment was sent to (if you paid that way). If no response, then I'd take it to a dispute.
