"Maine Joins Colorado in Finding Trump Ineligible for Primary Ballot" (source). Unlike Colorado, this decision was made by Maine's Secretary of State. Trump's legal team has appealed the decision to a Superior Court in Maine (source).
Resolves YES if:
The Kennebec Court Superior Court agrees to hear Trump's appeal
and reverses the Secretary of State's decision, stating Trump must appeal on the state's primary ballot
Other context:
Maine's presidential primary is scheduled for March 5th
Because Colorado's decision to bar Trump from the ballot was decided in their state supreme court, that decision can only be appealed to and reversed by SCOTUS. Because Maine's decision was made by an official, the appeal goes to a state court (Kennebec Court Superior Court, to be exact)
Edge case: prior to a decision from Maine's state court, SCOTUS could review Trump's Colorado appeal and respond by affirming Trump's ballot access. Such a decision could supersede a need/ability for the Maine state court to weigh in. In that case, they would not review or issue a decision on Trump's appeal, meaning this market would resolve NO on a technicality.