The sites do much of the legwork themselves by organising their own credit checks on borrowers and chasing up any overdue payments using a debt collection agency.
Chasing overdue payments causes stress and anxiety; it's a waste of time and it fundamentally stymies growth by forcing the business to focus on surviving rather than thriving.
Back this up with systems and processes that bring discipline to collections, by enshrining timely payment reminders, overdue notices, and proactive follow-up calls for overdue payments.