Well, the nexus 3 - part 2 clearly states a requisite can be conducting business with US.
It seems to me that if youre in the domain selling business trading with mainly US concerns then you do fulfil that requirement. If you can show that you've done logos or a website for a US company then its even better.
Showing receipts of USD transactions in your Paypal, Escrow or Merchant account should prove it.
Overall though, if i was you and I had some valuable .us, then I would open up an office, bank account and LLC in the States which is fairly cheap though you'd have the hassle of processing accounts.
Any large domain trader should do this anyway as it makes some forms of online business and payments much easier.