First of all, don't do it. Get the seller to provide the software Serial Number (the one on the card or slip of paper, used for Registration to gain access to drivers, control software etc), not the hardware serial number (the one on the label affixed to the unit). He/she will also need to Transfer that Serial Number to you.
Failing that, Technical Support are the only ones who may be able to help. Please contact them via here. There is a link at the top of the Search FAQs page to the Technical Support form, for some one to one investigation and help.
Response time aims to be within 2 working days (US (East Coast) Business hours, generally), but in the current circumstances please allow some more time.
The same advice applies to when you receive a software Serial Number but find that it is already registered to another user.
Posting about this problem in the forum will not help; all we can do here is suggest that you contact Support.