My experience with any of the lenders/banks has been that if they will pay any of the buyer's closing costs at all, it is capped at 3%. Perhaps if your offer is high enough, they may pay them, but you must be able to get a high enough appraisal for it to work. With the recent plummeting of values in some areas, it is hard to say if that will happen. Also, in some cases, some of the typical Seller's closing costs must be absorbed by the Buyer - such as documentary stamps on the deed. If this is the case, it may not work at all.
If the home is listed, and you are working with a Realtor, ask one of them what the lender will allow. If not, have the Seller talk to the negotiator to find out. If there is no experienced professional representing the transaction, it would be wise to have the Seller hire a professional negotiator to cut through the red tape.