I'd love to try this out, but a facebook login wall is preventing me from doing so. I personally don't have an account, but I can see others not wanting to connect their facebook accounts with their businesses.
I think when you're talking about a payment integration system, having to create an account is not what will stop "sellers", as you call them, from signing up. It's probably different for end users though.
(full disclosure: I'm a friend of the Willet devs)
Willet promises better conversion rates because it only takes a couple of clicks to get started. The Willet tab is shared across all websites that use Willet. As well, we promote your app / service to other Willet users to speed up the time to reaching their tab limit.
Really nice guys, I'm digging the new UX.
I guess it's better than Apple's terms, but still.
Waiting for a check is the norm because there are (unfortunately) costs involved in sending money. $100 is not too difficult to make for even a small app.