First, I would find out if the seller had the tines that were removed to make it whole again. Second, ask yourself if you are going to use it as the seller has it set up. For all we know he tried a modification and it didn't work as planned or it works great for his intended purpose, I'm just not exactly sure what it was. I wouldn't want to be changing that back and forth between rake and "whatever it does".
If it were me and he had all the tines I would offer him $550 plus he could keep the clamps and the added front bar. The $650 price doesn't seem way out of line.