Corrosion marks are usually not hard to clean. I revived this way a Metz CT-1 which I received from one of those dubious auctions on eBay, described as an heritage, as-is. After cleaning (just using a "brillo" pad with some alcohol IIRC) the flash was as new. It also happened on some other couple of occasions with something at home.
I have often read that corrosion on the contacts can damage the electronic components inside, but never experienced that.
I would write to the seller showing the pictures with the corrosion, and suggest I would try to clean the contacts by myself, but if that is not cure to the problem, I would send it back. Sending the photographs first is important so that they realise that the description was not correct.