Polished Limestone Fireplaces?

Hi! I have a question for anyone with a polished limestone fireplace. I saw one I really like in a showroom last week, at a really good price, only thing is you have to take the display and it has gotten a little grubby. I'd say whoever put it together stood on it in a few places so there are some grubby black marks, there is also a little spot of yellow (maybe paint) on the side of it. Obviously I dont want to buy it if I cant get these marks off, and the shop say that they are selling as is, if they go to the bother of cleaning it, they will also raise the price! My question is how do you clean polished limestone without damaging it, and also are you supposed to seal it? Is it really hard to keep clean? I'd really appreciate any feedback on this, Thanks.