Just a possibility here, but it's likely your seller has set up the Automatic Unpaid Item Assistant, which will open and close the disputes on their behalf, in general, it only works with electronic payments like PayPal, and shouldn't have been possible on an item paid for with PP, but there seems to be some sort of a glitch at the moment with eBay's records of payment and that may have triggered the dispute.
One of the items I purchased and paid for last week (which also has apparently already been posted), is now showing as unpaid. This was an instant payment with funds already in my account, and to a seller I buy from on an almost weekly basis, so there is no doubting the payment went through (I double checked my account just to make sure and the status is Completed). If they're a high volume seller, they probably just rely on Selling Manager Pro to tell them things like what's awaiting payment, what needs to be posted and so forth, so if their account is telling them, albeit wrongly, that an item is unpaid, they might not have picked up on the error - communicating this may be a problem by the sounds of it, but if worse comes to worse, you will be able to appeal any strike that's issued and have it removed.
Sellers can also manually unmark items as Payment Received (meaning they indicate that payment hasn't been received), but I don't know what happens if they do that when the payment was made via PayPal, and whether unpaid item cases can then be opened.
