well if you don't get it sorted it wont pass the next MOT , all dash lights should go out when the engine is running, part from the hand brake light (if the hand brake is on) get the seller to fix it, or get the price down

if he does if it, make sure it does come on went you first start the engine and then goes out, some bar stewards disconnect it or remove the bulb so it should pass the MOT if the tester notices, he probably will cos this law has just come in (all dash lights must come on and go out as they should)