Opened a trade, got card not in confirmed condition. Posted 2 messages to chat, also sent an email to trader; they've logged in several times in the past couple days with no response. At what point do / should I open a dispute or leave negative feedback? (The value of the card in question is <$10.) Not sure what's appropriate; I've not had this situation on this site.

Open a support ticket. Sebi will sort it out with the two (or just 1) of you.