Hi,
We've owned an apartment in block 2 since last November and we have noticed too the floor marks very easily particurary by black soles and suitcase wheels etc. We've had to use the sponge scouring pads to keep on top of it, yet we've had no renters. The problem is due to the fact that porcelain tiles are unglazed and non slip, unlike glazed cermaic tiles which are easy to clean but can be slippery. The tiles on our balcony have been stained by what we think is some sort of creosol the builders have applied to the render. (we can't remove it) I am a tiler and frequently visit a tiling forum so did a quick search for you and I think you will find the links below useful, particulary the last post in this link explaining why they mark. http://www.tileforums.com/index.php?showtopic=4435&hl=cleaning+porcelain
Also look at http://www.trades-direct.co.uk/modules/shop/products.asp?catid=21 and give them a call for advise on their tile care products for unglazed porcelain. Let me know how you get on, I might be doing the same yet.
Regards,
Steve