Venting of the kitchen sink should not be a major problem. Devices call "Air Admittance Valves" or AAVs can often be installed to vent an individual sink (this is frewquently done to vent an island sink, for example). These are inexpensive and can be installed by any qualifed plumber. A good site for more information is: http://www.toolbase.org/Technology-Inventory/Plumbing/air-ad
Hope that helps!
As part of the negotiating process, your Realtor could ask the seller to correct the problem prior to close. Since I don't have first hand knowledge of your situation, run the idea by your buyer's agent and see what he thinks.