The pictures aren't very good, but it looks like that could be the old-fashioned kind of post with a removeable clamp for mounting the saddle. If so, that's partly good news, partly bad. Good in the sense that I thought the clamp had been broken off in an attempt to get the post out, but that isn't necessarily the case, so it isn't necessarily that badly stuck. But bad in the sense that I believe almost all of them were steel.
I don't believe this line that the post is actually bigger than the tube and has been hammered in there. I don't think that's possible. But if it's a steel post, there could be a steel:steel rusted-in issue (albeit note that it's painted). If it is steel, melting it out is impossible and cutting it out will be much harder work than soft aluminium. The drill a hole and use a lever method may be the only feasible way, but at least steel shouldn't break the way Jerky said.
I reckon if it sells for around £150 and you haven't got too far to travel, you've got the frame for nothing anyway. And you won't find many of them come up on eBay. Maybe one or two a year.