The going price for a triple bench seat is no more than £500. So i’d be checking if the price in the posting includes the quick release foot brackets, the under chassis strengthening bars, and the fitting. For a triple you need 4 QR foot pieces. Ebay generally has these at £100 for 4. The strengthening bars probably about £50. You’ll then need the seat belts for the third row and the special adapter pieces to make the belt sit in the correct position above the shoulder since the top anchor point is on the side wall box channel right up under the roof. A second hand belt will be around £50, and adapter piece prob £20ish. All in all I would think all the bits would be £300. So fitting is £200? Might be a bit low in my mind, so definately worth double checking exactly what you would be getting for your money.
Send ZipcodeUK a PM and i’m sure he will give you an accurate breakdown as to the cost of each of the bits required as well as how much he paid for the exact same job to be done.