If the site purchase and contract to build/ sell are related contracts, then they fall within stamp duty remit. Remember also that anything that looks too good to be true normally is.
Purchasers may be able to avail of f.t.b. relief, or relief if they are owner occupiers and may legitimately not pay any stamp duty BUT if the purchasers are investors then stamp duty will be paid at full value at full rate.
If there is no connection between the vendor of the land and the builder and if the purchaser genuinely employs the builder to build, then they are not related contracts.
I don't think its ingenious.
mf