As a general rule, we do ask that you don’t add buyers to mailing lists unless they have specifically opted in:
In using Folksy you agree not to contact your buyers unless they have opted-in to any further communication (through any channel - phone, mail or electronic).
You can ask your customer if they would like to opt-in to marketing based communications from you at the point at which you deliver the goods, for example in any literature you send with your item (or if they were to contact you for any reason), but you must not assume that they have opted in unless they have expressly chosen to do so. No one likes unsolicited communication.
It’s also less likely your emails will get marked as spam - which can sometimes lead to ISPs blacklisting you - if you ask first!
Mailchimp* (*other mailing list services are available) have lots of guidance on best practice: