I'm going to try to verify this week that a Sole Proprietorship with an EIN is even feasible. The reason I want to investigate is to get multiple accounts with each CPA net to spread the risk out for blackhat traffic.
If I declare myself as a sole proprietorship I call "Acme Marketing" and get an EIN, I can open an account with a CPA net, giving them the name Acme Marketing, the EIN, a UPS Store mailbox and a skype phone number. The CPA Net would have no reason to link that account to the one that's in my real name.
I've currently got 10 CPA networks, and am aiming to grow that to 25. If I then get a box/phone/EIN I can double that to 50, there are at least three places within 5 miles of my home where I can get a mailbox, that would take me to 100.
Why? Because blackhat CPA is risky. It's far too easy to get a complaint or to let something slip and have an account get locked. With 10 accounts, that would be 10% of my income stream. With 25, it's 4%, with 100 it's 1%. I've got systems in place to rotate and track offer URLS so that I can ensure that traffic is spread out evenly. A bit of tracker202 trickery for some referrer scrubbing, some work to distribute out my DMRs out over several seemingly unrelated boxes, and no single CPA Network account can impact me enough that I feel it. At least that's the theory.
Why not LLC? Eventually. But LLCs cost money, require crazy paperwork in some states, and are a lot more initial work to set up and maintain, whereas I'd rather get things moving.