Let me say first that I could be wrong about any of this, but according to the user agreement section 20 (g), we're independent contractors, ie, freelancers, not employees of sharecg. Which I don't blame them for in the least from a liability and responsibility perspective. I believe this is how the various poser markets do it as well.
As freelancers, as it spells out clearly in the user agreement, and I've seen spelled out elsewhere before, the whole taxes thing is already covered by stating bluntly that it is Our responsibility to declare the relevant amounts and pay the appropriate taxes, not theirs to do the witholding thing. Which AFAIK is standard for a freelance situation. Freelancers operate under entirely different rules than normal.
I do understand what you're getting at by suggesting that everyone has to give an SSN etc, it would eliminate a number of problems. Since no one who wants to scam, would want that kind of identifier. That being said, I think you'd find rapidly that a lot of people who Aren't scammers wouldn't be willing for the following reasons:
a)because it's not legally required by the relationship, or it's illegal in their country
b) Because if sites with a heck of a lot more operational budget than Sharecg can't keep data secure, they're going to Severely doubt Sharecg's ability to do so. Identity theft is a very lucrative business these days as we all know. One hack, is all it takes.
c) That whole issue that people are told every day Never to give out that kind of information online.
What I think Would work, and no one could complain about, is requiring that people use only Verified Paypal accounts. To have a Verified status, it's required that you have verified an email, as well as a credit card or bank account. Do you think scammers will want to do that?
Both ways, have issues of course. A scammer could I'm sure easily find fake or stolen SSN's, just as they could find a fake or stolen credit card. But requiring a verified paypal account, would make a lot of people a whole lot less nervous I think.
Personally, I tend to favor the opt out or two tier system.