It's such a huge gamble, please don't do it. My inlaws have just sold their house, finally, after four years on the market, please don't give up hope just yet. I have moved 12 times over the past 21 years, and the stories I haveheard along the way are just awful. Some estate agents really can't be trusted, most of them, we discovered, will say to the buyer 'they are asking for £300,000 but offer them £280.000' because they want a quick sale and the commission - most are on commission only.
I bet if you do what he wants, he will be on a very big fat commission from the builder. Your house and land is probably worth far more as building plots, which are extremely scarce. Your house may be hard to sell, but knocked down, with a whole load of small terraced properties on it, it would be a goldmine. We tried to buy a 1/4 of an acre plot in the inlaws' village, for just one house, but were outbid - the developer managed to squeeze a dozen homes into that space.
This made me quite teary - but smile too

. My fingers are crossed for you.