As sdr said there is a proper way to go about renegotiating that has to do with timing. If you are simply trying to renegotiate the sales price for no reason other then to run the price down, then you will most likely piss the seller off and can possibly be liable for liquidated damages. However (again as sdr implied) through the due diligence period you found that the home was in need of serious repairs, or there were disclosures that pointed out things that were of true and real concern, then yes your request would be received in a much more realistic sense by the seller. Again, many times there are requests you will want to make, that may be received in a much different light, by the seller if you have good reason, and time them properly.
On the other side of the coin, a buyer just tried something similar to that for one of my listings. I feel we have the price set very aggressively on the home, and that it would sell if we re-entered the market so we rejected his offer and delivered cancellation instructions to illustrate our resolve that he could pound sand. He did not cancel and has moved forward with releasing contingencies.
