The old account would usually be removed when the owner is getting it ready to sell. Where did you buy it from, was it a legitimate retailer because it seems a little fishy to me like it could be a stolen phone but its not impossible for the owner to forget I suppose so in that case contacting the seller who would know who it belonged to and the original account holder would be more than happy to have their information removed from something they sold. Maybe samsung can comfirm with the account holder.
Just had a thought of easy way to sort it, if the old account is on there I'm assuming its showing you an email address of the account. Email them and tell them you bought the phone and their account is still on there. If it is a genuine mistake and forgetful previous owner they will be able to log into their Samsung acccount using findmymoble to backup and remove/delete accounts and data remotely leaving it ready for login of the new user . If on the other hand it is a lost or stolen phone you could be helping someone find a missing device and then its just a case of taking the right actions to gettiing a full refund from the retailer which shouldn't be difficult.