In regards to the post about money:
Yes, it is a "gifting" system, in that everyone should bring everything they need, and then they gift to each other, things like art, nifty little trinkets, food, etc. You don't actually Barter, per se
You need some money, because there is an ice tent where you can buy a block or bag of ice for 2-3 dollars (and you'll need ice to keep your food cold, or ice for drinks, etc). Also, center camp has a coffee/tea stand that takes money. Besides that, nothing takes money.
Money is definitely used to buy illegal drugs, i pretty sure people don't gift drugs that much. You can get absinthe gifted, and i've been gifted plenty of weed there (but was gifting in return), but harder stuff you usually have to bring with you and buy.
Booze and beer is gifted like crazy (half or more of the camps have bars that serve freely without taking anything in return, but its nice to gift them something cool). Many art cars also have bars on them, and don't take anything in return for drinks. Be interactive with them when they are serving you, socialize and have fun, its part of the experience

.
In regards to police presence:
Of course there is police around! Everyone from local police on up to feds (it is bureau of land management land, after all!!) Don't be stupid and walk around using/yelling about drugs in communal spaces with a lot of traffic, you will get cited or arrested (just like any big city). Usually there are a couple of hundred citations made a year. That said, police don't go around to private camps, they're mainly out on the playa, and at the big communal party camps, patroling. There are also burningman volunteers, Rangers, who act as safely officers. They're awesome people there to make sure people are being safe and following the rules, and they don't get police involved unless they absolutely have to. They patrol burningman, protect people from the big burns (burning the man, etc), and prevent people from sneaking in (they have a radar that can see things as small as small birds out past the limits of the city

). They volunteer 50+ hours for a couple of weeks making burningman the safe and awesome place it is.