Ask HN: how to verify users identity (background check)?

As I am building my marketplace startup, I would need to verify the identity of my user before I send them ACH for their payment.

I am curious to find out what and how you would use in term of verifying your user identity. I understand like airbnb is using offline identity (passport or driver license), taskrabbits are using background check (sterling info systems), I think Zaarly is also using some background check as well.

if using background check firms, who would you recommend that is startup friendly in term of integration and cost?

Thanks.