I have one MSC that I regularly shop for. They are one of my favorites. But they will not accept an EIN.
I have recently done my first shop for TS. Although their sign up procedure says you just don't get to self-assign without an EIN, their inital quiz has a question that suggests that you won't get paid without an EIN. Nice of them not to disclose any of this up front - NOT!
I can probably do a lot of business with TS as they seem to have a lot of projects that come up in my area - like 15 shops less than a mile away in the past week. So it is worth considering getting the EIN.
BUT - what happens at tax time if you have income under both the EIN and your SSN? Do you have to file two separate returns?