You buy Marketplace points at your local store (right now they just have cards with 1600 points on them) or over Live. Live has a lot more options, the biggest option being 5,000 points for $60 I believe. You buy them though, and then you can buy anything on Live with them. Arcade games, gamer pictures, themes for your Dashboard, downloadable content, etc. Trailers and demos are free. Some things are being over charged for already... Bankshot Billiards is 1200 points to buy, which is ridiculous. Gamer pictures run about 20 points and themes usually 150 or so. Eventually you're suppose to earn Marketplace points from achievements, but Microsoft has to work out some legalities in some countries and such before that happens. Also you'll be able to sell stuff over Live to people for points... and sadly, people will be stupid enough to buy the most simple of things.