[chrisjj]

> So the question here is what part of their system is so fundamentally broken that scammers instantly get my email?

Perhaps none. Did the T&Cs permit this disclosure?

[hardenedmetapod]

Not that I can see offhand. It mentions using your email for correspondence and copyright disputes.

[chrisjj]

I'd say odds on Poshmark leaking your address to the seller.

The fact you got spam so soon makes me wonder, did you get your goods?