How far in is the seatpost when it binds? When I worked in bicycle shops the rule of thumb was 2 inches inserted BELOW the top tube. Some seatubes have a fair bit above the top tube and if the seastpost isn't inserted to a couple of inches BELOW the top tube the lever effect of the seatpost and rider can snap off the seat tube. I seen a number of frames where that's happened.

Is your post hitting a seat tube bottle mount?

Is the interior of the saet tuber rusted or is the seat tube dented?

I bought a bike that had this problem but it was in the steerer tube. The stem would go down but not as far as i wanted. Took out the stem, shone a bright light into the steerer and found that someone had cut off a stem and hammered the section left down into the bottom of the steerer. Fortunately i was able to bore out that piece.

Cheers