On a used bike you might check if any bottle bosses or other mounts on the seat tube weren't added by a previous owner and it's not just a rivnut or screw going into the seat tube and seat post.
Failing any joy with penetrating oil, I'd probably take the saddle off, turn the bike upsidedown and tighten down on the seat post with a bench vise and twist the frame to see if that will loosen it up.
Yes at that point I'm assuming the seat post is trash. So tighten the heck out of the vice
If you don't have a bench vise then a long handled pipe wrench will be my other choice.
Are you certain the seat post is steel? I'd think aluminum. But then again I am surprised a 15 year old bike only has six cogs on the rear. You probably paid 35 dollars too much.
Going back and seeing you did say cruiser bike I can imagine 6 on the rear. Personally I think from the turn of the century to just a few years ago manufacturers lost direction with cruiser style bikes. Seems like they have been getting better with them lately.
Last edited by Iride01; 06-13-22 at 10:35 AM.