Can I buy a post ban receiver and a pre ban kit, just as parts and not assemble them until Sept.?
Is that legal? Afterall, I'm just buying parts right? There wouldn't be atf agents breaking my door down would there? Any advice or experiences would be helpful, thanks.

Sure!

It seems that the general concensus is that if you have a COMPLETE postban lower and a COMPLETE preban upper, the ATF can bust you (if they wanted to) for "intent to build", as it only takes seconds to put an upper on a lower and have a complete rifle.  It may be a different story if you have two UNASSEMBLED pieces, the unassembled upper and unassembled lower.  I am not sure about that, though.

Your idea would not be a smart one, just wait until Sept. Why even chance something like this?

Be smart.