Your first course of action should be to return the system to the seller for a refund.   Then report the suspect system to Dell here =How to Report a Dell System Lost or Stolen
I purchased a Latitude 7420 from eBay. Upon reimaging, Windows defaults to a corporate logon. I now suspect that the laptop was stolen or otherwise misappropriated. I can provide service tag, express service code, company name and the seller's information. Local authorities suggested that I contact Dell to contact the company rep to verify whether or not it was stolen. 
Possible Stolen Laptop.