Hi Pam
An underbite is obviously a big no-no for anyone wanting to show and/or breed although it's not particularly significant for a pet dog unless the problem is severe. Another member of this forum was recently offered a young liver roan with a mouth fault - there were other problems as well which came light when the breeder/owner was visited & so this member declined to buy this dog. I'm not saying the Cocker you have been offered is the same one, but just in case it is, feel free to contact me privately & I'll give you more information.
Jane
PS I would not expect a Cocker with an undershot mouth to be offered for sale at the same price as one with perfect dentition.