For $400 I'd buy it and put a new used tranny in there (assuming the engine is good and other systems are fine). Or if you are feeling up to it get/do a rebuild on the current one. The interior is a bit rough but if the miles are under 300k and everything else on the car is fine you are getting a good deal. But I'd still offer $300, never offer full price. You can always pay full price but most sellers start high and are willing to come down on their price.
If the engine is in decent shape, buying the whole car for $3-400 you are already ahead as the engine is worth about that.