ah.. so not the phone owner then? you have some options. 1st, does that individual know who the owner was before them? if yes, ask them to ask that person... a) do you remember what Google account you used with the phone?
b) if yes, have you changed your password?
c). if yes... read below
once they do, and it changes through the whole account it'll toggle it off most times but can take a bit, I've seen 30mins and 5days, Google says 72hrs for anything with them. I'd say 1 full day is normally good, and you'll prolly have to manually factory reset. do you know how? if not I can tell you.
d). if no to the password, have them go into their Google account and remove that phone from the account entirely. remove from trusted devices, saved devices, connected devices, the number used on it, and the 2 factor authentication method of text codes. or, if they know how to, go either adjust there whole account sync settings so it'll be forced to lock/unlock 1 time, or turn it off if they know what to do.
no sweat if they don't know, that is just an easy way for a fix without out needing them to sign in to the phone again.
while waiting... what carrier are you thinking of using? can you get phone ins for that phone? don't tell them about the issue yet. yes or no, do they have a mobile security company they use?
those things are easy and quick to find out.. question for you?
do you know how to power on the phone into the recovery bootloader menu? do you have a computer or external storage device or another galaxy phone that has and/or used smart swtich?
