EBay invoices are sent to your NAME, usually to the email account that you have registered with eBay.
There is a transaction number on each listing. It is also called a listing number.
If you have paid by Paypal, they have their own numbers for each transaction they handle. This is on your Paypal Overview page along with the amount you paid and the name and address of the seller.
If you did not find the item on eBay, or if the seller wants to send you an invoice separate from eBay, do not fall for the SCAM!
Report the request to the site where you found the item. They will take steps against the scammer.