Having used a handful of sites like freelancer.com and elance.com with varying results, I'd say the best results I've seen were from postings I put up on freelanceswitch.com job board (jobs.freelanceswitch.com/jobs)
Adding features to Sponsorist.com, need a UI/UX expert to help layout new signup flow (preferrably 1 page), profile pages, embeddable widgets, etc. Need to keep it in Bootstrap. Please email info at sponsorist dot com for more info
"Paypal doesn't require this. A buyer can use their credit card. They don't need to sign up for anything."
This is not exactly true. When a buyer uses their credit card they are promoted to add an email address which essentially creates a Paypal account for them. They may never actually log into that Paypal account and use it, but they will have a paypal account with the info they've entered. It's Paypal's sneaky way of signing people up.