That probably means that the lock needs a good shot of something like penetrating oil to get it freed up.
A locksmith could help.......and if all it needs is a little lube, they might not charge you anything either.
Did you get a second, spare key ?
If not, you definitely should hit the seller up for that; also an owner's manual.
I always carry the hex key to take my passenger seat off in the bags and tape a spare ignition key to the bottom of the passenger seat. Never lost my ignition key yet but only takes once to be thankful for the spare!!