Our QA team is currently investigating the issue with the messaging. So far, it is confirmed that attachments (such as images or icons in email signatures) are breaking some messages between members. Would you mind shooting a copy of the email response you were attempting to send to firstname.lastname@example.org?
In the meantime, I'd be happy to connect you directly with your interested buyer to help facilitate your sale(s). I really apologize for the inconvenience this may be causing on your end and thank you for your patience as we work to resolve this ASAP.