Its the old story, "you get what you pay for", any one that's done their homework will know its ?600 to ?900, for a belt change.
Plus the overall condition of the car, bangs, dents, tyres all make a lot of difference. I would pay a lot more for a decent, tidy, up together sample, over ones that's had a hard life.
The advert will sell the car, plenty of photos and a history of the service, well worth waiting for the right buyer.