But... the neg says the item was broken two days after and the vendor did not want to do anything,

If everything the OP says is true, the complaint the buyer had, the demands they made and the neg they actually left are worlds apart. i.e. If they thought they were buying a full set and felt mislead, why not leave a neg to that effect?

If the buyer never claimed the goods were broken, thus - even if they were - not even giving the seller the opportunity to do something about it, then if all else fails I would go the defamation route.