Notify the seller, and negotiate to return the note for a refund or make a settlement, even if the seller isn't active. You made the deal with the seller, not with the operators of the platform it was purchased through. Don't simply surrender and be unsatisfied. Unless the seller's idea to start with was a ripoff, most sellers will try to make the deal a good one. If ripoff appears to be the case, notify the operator of the seller's platform, with the seller's name. It may go after the seller on your behalf. You may also have recourse via the method by which you paid (credit-card company/bank/etc.). It's your money. Get it back if you're not satisfied.