First some advice for quoting in posts:
I've deleted the quote in your last 2 posts. It was referring to my post directly preceding your post and did not focus on any specific points that I made. In such a case quoting is not recommended as it only bloats your posts and makes the thread less readable for other visitors.
If you refer to specific points or refer further back in the thread, quoting is an appropriate tool for reference.
So please, use it wisely.
As to your question: The front-/backfocus problem is a very tough nut with regard to arguing. I once heard from a Tamron rep that this was not his problem but the problem of my camera
So the best is to buy at a shop where you can give the lens back with no questions asked. If it is a good shop you should discuss your problem with them to find out whether they would give you a second sample to try it out.
Thomas (beware: Nikon-fanboy and moderator!) My Lens Reviews
, My Pictures
, My Photography Blog