Valerie spots a rosewood bookcase in an antique shop owned by Jim and Belinda, which has a price tag of $4000. Valerie approaches Jim and tells him that she will take the bookcase for $2000. Jim says that it isn't possible but how about $3000? Valerie says no, how about $2,500? Jim says no again, and he tells Valerie that his lowest price is $3000. Valerie does not reply that she will accept the bookcase for $3000 and she steps away from Jim to take a call on her mobile phone. While Valerie is talking on her mobile phone, Jim turns to serve another customer Isabella. After finish talking on her mobile phone, Valerie then turns to pay for the bookcase, but Jim says he has already sold it to Isabella for $4000. Valerie is furious and says that she and Jim had a contract, and the bookcase was hers.

As between Valerie and Jim, who accepted the offer and was the acceptance valid?