I know W9 but i don't know any other parking provider to ask for it (especially for non us residents) so far and also Sedo didn't request anything like this as most of the people here will agree
I know covering your butts is certainly a good thing but none will go easily signing up a service asking so much for their own protection
As you said IRS recommends non US people to sign and fax this statement, i don't know if they require it
I will tell you for sure that this will decrease your signups
I saw this in web hosting industry when they asked for photocopies of credit cards and signed order with fax. Most of these services had very low % of new orders
Regarding fraud i don't think a signed paper will be the one that will stop it
How you can be sure that the address is real ?
Take the place of a new domainer
See your service first and try to sign, doesn't know much about your service, see the requirements and leave
Discover trafficclub , see that behind this is a known verified registrar moniker and sign there pretty easy
In any case i wish you good luck with your service
Something else that saw at TrafficClub and looks weird
"- Affiliates will not be credited for traffic that originates from countries where the primary language is not English."