people make mistakes.

If it was me, I would reply with “You have not bought anything from me.  Please check purchase details and contact your seller”.  I wouldn’t report them based on one message.

If they message again tell them to open a case through eBay or PayPal.

If they persist then just ignore them.  It is not like they can open a case for refund or return.

If they really bother you then come back here and someone will have posted how to contact eBay (it is so long since I have done so that I don’t want to post misleading information [as some do]).