The Seller is right that it will fit 200 and 400 series tractors and the asking price is reasonable. However, if it is a 38" deck, then it would too narrow to use on a 400 series tractor effectively. You would driving over uncut grass with all four tires and trimming along side buildings and planting beds would be impossible. Before buying....ask what the model number is or have the seller measure across the outer blades to get the true cut width.
It has to be either a 38" or a 44". Pretty hard to measure and make a mistake over the the six inch difference.