If groups do that with groupcoin. Isn't there an arbitrage opportunity by mining or buying GPC? How do they value the GPC high enough to avoid arbitrage?
Dunno what you mean, but generally in the game(s) one just uses the "price list", much like when you go into a branch of your bank and buy or sell foreign currency; though maybe I am way out of date, as I am recalling from decades ago my bank would just look up the rate for USD versus CDN or whatever, not go log on to some forex sites and check for arbitrage etc.
Basically there are a bunch of tables and plots at http://galaxies.mygamesonline.org/digitalisassets.html
and the latest figures added to them get put at http://galaxies.mygamesonline.org/latestrates.inc
which is formatted suitably for use as an include-file in shell scripts and such.
For example these latest rates figures were used in a bunch of market-making scripts that would add or subtract one two or three percent or whatever to make offers on three scales of the Open Transactions server's markets. Actually you lot would likely even think the way those scales were done is kind of the opposite of what you are used to since the larger the scale of the market the less the markup or markdown, so that to get the best prices you would buy in bulk. So for example someone could buy something in lots of one million and break that up into lots of one hundred throusand or even ten thousand to sell at a profit on a smaller scale market. Sort of like bank rates or nation to nation rates versus wholesale rates and retail rates or whatever.
So basically when you look at http://galaxies.mygamesonline.org/latestrates.inc
bear in mind it is a middle of the road figure, sellers would mark it up one, two or three percent, or maybe more depending on relative scale of the assets of a pair, and buyers would mark it down, so typically you'd be offering to sell for more than those prices or offering to buy for less than those prices and how much more or less woudl depend on the scale of the market, basically you'd be asking more for screwing around with just hundred-lots or thousand-lots than for making substantial trades at larger scales.
Also bear in mind that the Digitalis server only uses integers, shares or coins are all the same, commodities measured in integers, no decimals/fractions. So in addition to the markup or markdown then also there would be rounding to be a price in integer number of this per integer number of that.
The whole numbers even for currencies is not only to stop acting like currencies are somehow magically different from shares or commodities but also because in the kinds of games a lot of players were used to one uses whole copper pieces, whole silver pieces, whole gold pieces, whole platinum pieces type of coinage not "pices of eight" as in chopped up little fractions of coins. So basically this way we get all assets to be similar in their integer wholeness whether whole shares or whole coins or whole porkbellies or whole pallets or containers or shiploads of porkbellies.