Counterfeit notes are more and more of a problem with collected online purchases. Sometimes the top one is genuine, the rest fake. The seller gets aggro from the bank becoming suspicious of them. Also, increasingly common, and maybe what is happening to the OP, is this
Initially the scammer will send a message to the seller pretending to be interested in their item. They then claim they will pay for the item by sending cash in an envelope which will be delivered by a courier company such as UPS Delivery or TNT Express.
Often the scammer will claim to be stuck in work, or unable to drive, hence the offer to send the envelope containing cash through the courier service. If the seller agrees to the unusual method, the ‘buyer’ will then ask for their name, address and email address.
Once they have this info, they will ask the seller to pay some kind of ‘insurance cost’ to the courier. The ‘buyer’ will try to explain that this is needed because the courier is shipping physical cash, but not to worry and that they will get this money refunded.
Meanwhile, the seller will receive an email appearing to be from the courier company, asking for the insurance cost to be paid so the courier company can make the delivery.
This will NOT be a legitimate email, no delivery will have been arranged, and the link in the email will direct the seller direct to a phishing site where the money will get paid to the scammer. The phishing site can also steal the debit/credit card information that is being used to pay.