The seller should have followed these instructions: What to do before you sell, give away, or trade in your Mac.
Apparently he or she was remiss in preparing that Mac for sale. Ideally, you should return it in accordance with eBay's policies. If you don't want to do that for whatever reason, perform as much of the above instructions as you are able. You will encounter continual problems with that Mac until you do.
99% of eBay sellers fail to properly prepare used Macs for sale, resulting in misery for buyers and sellers alike. Insist they do it right.