Since secret shoppers have to file W9 and taxes to the IRS, will this prohibit international student (F1 visa holder) and employee (H1B visa holder) from participating?

I'm honestly not sure about that - perhaps you can ask a tax accountant? Do some other members of the community know more?

Yuan I suggest calling INS, Immigration and

Naturalizaion Services. I'm pretty sure that they are going to tell you no, unless you are able to file a W9 with the MSP is you choose to work with, and some will require it to be filed. Other MSP's might not. You might also consider contacting each MSP and ask them, too. Hope this helps.

Thanks Norm!