Probably the starnut or bolt were damaged or stripped when the prior owner tried to adjust the headset without loosening the stem first. Or possibly there's no centering cone, so he forced it, but it still couldn't tighten.

That's just speculation, but probably not a mile off, and it would have been nice if the seller mentioned headset issues. but thats water over the dam now,

Step one assess your situation. Remove the bolt (if it'll screw out) and lift off the top cap. If the bolt won't back out, you can sometimes get it started by using the loosened stem to put some upward pressure on the top cap as you try to unscrew the bolt. If that fails you'll need to resort to extreme measures to remove the top cap (if this is the situation, post anew, and I'll give you some ideas)

1- is the stack height enough so the top of the steerer is recessed below the stem or top spacer or by 2-5mm? If not you need another spacer?
2- is there a centering cone for the top bearing, and does the lowest spacer, or the stem press on it without touching the surrounding top HS part?
3- is the star nut OK, or stripped, or damaged, and is it broken enough to come out (they usually aren't)

If all is OK except for damaged starnut stuck in the steerer, the easiest solution is to use a punch to push it about another inch deeper and abandon it there. Put in a new starnut with a new bolt and you're good to go.