i agree with velveteen, but i do understand why people buy new, everything is done, easier to keep clean, just move in, no work required !
i sold last year, it took ages, and i had to reduce the price. so selling to the builder is a good thing and he will give you market value of yours. i think new house prices are over inlflated, but if you are going to stay for a long time, better than struggling to sell yours, the way the market is, the builder may be desparate to sell and give you a good price.