You can negotiate as much as you want while making an offer/counter offer. Then while working through contingencies. After all contingencies and checked off, you should not make anymore demands, unless you find a new defect in the structure. Any negotiating after that would be considered rude. You as a buyer would have to decide for yourself what you think is the moral thing to do. You would only be risking your deposit. You could also be subject to a lawsuit, but this rarely happens and is not worth it for the seller.
