Read the Sheldon Brown guide on stuck seat posts, work your way through the various option. The last one is cutting it out by hand. Take your time if you go that route. Search this forum, as there are many reports on loosening up a stuck seat post. I have freed up about a dozen bikes using the various techniques. I still have one that will not budge, hanging in my workshop, waiting for me to cut it out. I figure as long as I have projects around without a stuck seat post, I will work on them first.

Just freed up another post this AM. Getting a rash of them lately. Fortunately, I ALWAYS check before I buy a bike, and when the post is stuck, I base my offer on the value of the components (in case I do not have any luck freeing it up). Once the seller realizes the post is stuck, they are usually pretty open to an adjusted (lower) offer.