>how to run their selling thing
Go to this page: http://g.e-hentai.org/exchange.php?t=gp
Look to the right column, that's for people selling GP (to get credits)
Under sell count, enter "1000" (that's 1 million GP).
Under "Ask Price", enter 168. That should be a good value for now.
Click "Sell GP!"
It's basically a stock exchange kind of thing. What this does is put in a sell order; you let people know you have 1000 KGP, and are looking to sell it at 168 credits per kgp. If someone wants to buy any amount of GP for a rate of 168 credits/kgp or more, you'll automatically sell them that GP and get an appropriate number of credits. It won't necessarily be all at once, you might get something like 5 kgp to one guy, 100 to another, etc.
To get a real value for sell price and not just use 168, you look at the spread. That shows everyone's current buy/sell requests, grouped by the number. Right now the most expensive buy order is 167 credits, the least expensive sell order is 169. Putting in 168 will mean you're first in line for anyone trading at that value. However, since it's a stock market kind of thing, the prices fluctuate a lot. You might just get someone else wanting to sell 10,000kgp at 167, for example.