I don't understand your last post at all.
What is it you intend doing, is it undertaking a once-off buy from plans and then sell before completion, or a series of such transactions?
If it is a once off, then the 'only' downside is the loss of FTB status. But as was said earlier in this thread, you need the developer and solicitors etc. onside for this to work. There should be no stamp duty implications, but you may be liable to CGT on any gain (less allowable costs).
When I talk about the Revenue taking an interest, I am referring to a scenario when this would be happening on a regular basis, and would be deemed a 'trade' and hence attract income tax on the profit generated.