I have won an item but suspect a shill bid raised the price by EUR85 in the last 2 secs. Should I continue the transaction?

[sarahs_artworks]

I have won an item where a bid was placed 2 seconds before the end of auction from an unknown bidder with no previous bids on the item. This bidder bid an identical amount to mine, raising the price EUR85 in one bid, where all previous bids raised the price in much smaller increments. This bidder also has a track record of placing a single bid within the last hour of bidding on many other auctions associated with this seller. I have requested that the seller explain this action, and compromise a sale with me. I have also reported the action to eBay using their report forms but am aware they can not inform me of an outcome. Now I have been issued with an "unpaid item" case requesting payment within 4 days. What can I do?