Sight unseen I can not say for certain but it sounds like a common scam. A "buyer" sends you an email proclaiming payment. Did it come to your personal email or is it in eBay messages?
In the seller hub, in addition to "tasks" there is also a block headed "orders". Look to see if there is anything on the line "awaiting payment". If a buyer clicks "buy now" it would remove the item from your active list. They then send a false email claiming payment has been made to get you to send the item. Only no actual payment has been made.
Can you find the record of payment on your managed payments page?
Until and unless you get something FROM EBAY and through EBAY MESSAGING and MANAGED PAYMENTS you did not actually get paid.
Ignore and block the ID.
"Laissez-faire capitalism (AKA The Great Material Continuum) is the only social system based on the recognition of individual rights and, therefore, the only system that bans force from social relationships." ~ Ayn Rand
