Please consider adding a feature to MarketPress to subscribe purchasers to your MailChimp mailing list.
I imagine it would work like this:
as user proceeds through checkout, there would be a checkbox asking to subscribe them to the mailing list. if they leave it ticked or tick it, then they would automatically be subscribed via mailchimp.
I see quite a few questions on the forum asking for how to achieve this so please consider integrating a solution into MP.