ShipScript has been an eBay Community volunteer since 2003, specializing in HTML, CSS, Scripts, Photos, Active Content, Technical Solutions, and online Seller Tools.
If a buyer sends an eBay message from a listing, the seller can usually "reply with offer" to that initial eBay email, even if they have already responded to the message. This listing must still be active and can not be an auction or a variation listing. Subsequent messages (those that start with "Re:") will not have the "offer" feature.
