Jerking while idling on 2005 Nissan Sentra

Just bought this car from a dealer 4 days ago. Today I have noticed some jerking
or quaking especially when idling. Could this be a bad spark plug? I'm hoping for a simple solution and that they didn't sell us an expensive problem.

Asked by for the 2005 Nissan Sentra
could be many things - take it back and have the dealer look at it.