If you're registering these dot.us in your registrar account but using his address for the registrant info... you're in for a heck of a mix up if any name, ever, has any legal trouble. Not to mention it's a little fraudulent, in a way, though I don't know the legalities of this; but you are placing false information in your registrar account. Further, if someone wants to buy your names and does their homework and finds out you have more than one registrant address, this might be enough to scare them away. And yet further, if there is any investigation, it'll be seen that you're not American and have no 'activities' in the US (assuming you don't).
Those are bad-case scenarios.
A lot of people do it and get away with it just fine, using the addy of some buddy in the US. I'd recommend a better way... hook up with someone who you TRUST (has been proven trustworthy, not just a 'bud') from the US, and have him/her reg the names in their account, after you send paypal funds to cover this. If the names sell, then (if they were indeed trustworthy

) they'll pay you according to your agreement.
If it matters, for what it's worth... I was tempted to do this, as I find a lot of dot.us names I'd like to get... but have never regged one. Just seems like too much hassle for too little return, large probability of problems and headache, and didn't leave a good feeling in my stomach.
GL