FutureBlock offer and example seems to violate No Arbitrage Rule If you look at, e.g. the best offers at the 420 BTCUSD strike price (this was as-of 4:15 pm EST on November 14, 2013), or at the "How It Works" page for Future Block, it seems like there is a violation of the No Arbitrage Rule.

For example, from the "How It Works" example. Suppose I sell one BTC's worth of "up" contract (I get 1 BTC) at 80% payout (I pay back 1.8 BTC if price is above strike at expiry). 

Next, I sell one BTC's worth of "down" contract at the same strike price (I receive another 1 BTC), at 50% payout (I pay back 1.5 BTC if price is below strike at expiry). 

If the price is above the strike, the "up" contract clears and I received 2 BTC but pay back 1.8 BTC (netting a gain of 0.2 BTC for me). If the price is below, the "down" contract clears and I receive 2 BTC minus the 1.5 I pay back (netting a gain of 0.5 BTC for me).

Obviously, these example offers may not be close enough to the best offer to attract buyers, but apart from that I can't see any specifications for transaction costs to the seller, fees, etc. Even if there were fees, wouldn't they need to adjust according to the payout rates a seller could command selling on both sides of a strike price?

I think I am probably missing an obvious detail; any help correcting my mistake would be great!

Added:

To clarify, the puzzle here is that at about 4:15 yesterday, the best offer for up and down contracts roughly matched the example I give above, using numbers from the "How It Works" page to illustrate a Dutch book offer selling on both sides of the strike price. That this can happen is not a mystery per se, but that the live best offers reflected that it was a viable live strategy even when there was resonable volume in the market is a puzzle. Either there was simply no volume at all on one side of the strike price (which should have immediately led to better offers on that side) or else there was actually not much participation and the volume data was incorrect / misleading, or both. (Assuming that early adopters of BTC options can't be that irrational... after all, they'd have to know a lot about bitcoin and that particular options platform even to participate. So it seems implausible that buyers were so irrational as to be buying at such prices on both sides of a strike price).
It is a well-known observation that Bitcoin exchanges are not efficient (in the academic sense of well-approximating what you call the No Arbitrage Rule). The reason is simply that there are real world barriers that academics usually assume not to be present. If you do consider the risk associated with depositing funds at a Bitcoin exchange, there is an obvious reason why someone would demand a high return for taking such a risk. If you look e.g. at the difference in exchange rates between big exchanges (try Mtgox, Bitstamp, BTCE) you'll see that a difference as large as 10% can persist on the timescale of a year.

The risks include having the exchange hacked and leaving it unable to return your deposit (have a look at Bitfloor) or having its bank accounts frozen during legal investigations and hence unable to return your deposits during the first 6 month or so (check bitcoin-24.com). And then there's the combination of it all with (presumably) enough reserves not to shut down the operation right away (e.g. mtgox). And that's not even considering the risk that the operator is dishonest and gets tempted to defraud customers...

On top of all this, consider that even moving Bitcoins around is not instant. You run the additional risk that during the transfer time the situation changes (e.g. a crash in Bitcoin exchange rate). And the transfer time between bitcoin trading sites can be surprisingly long, since some legislation (e.g. anti money laundering) typically seems to compel them to insist on verifying your identity. This process may, depending on the site, occur in a somewhat surprising fashion, so you never know if they will instantly, or at least within the first days, allow you to continue accessing your bitcoins (or real money) that you gave them.