>I'd be curious to know if anyone here can come up with a good enough reason for sending out the user's email & their password(-prefix) at every keystroke?

I wonder if it ties into their fraud detection systems somehow.

Fraudsters are lazy - so lazy that, for a good long time, you'd see the exact same few recycled photos of counterfeit items being used in item descriptions. No idea if that's changed recently.

Anyway, going back to my main point: I wonder if something about password entry and email address choice serves as an early warning flag.

I'd kinda be surprised, but I could imagine it potentially being useful.