The sigil trader is just like the dye trader or the heavy materials trader. Players sell sigils, dye or materials to the trader, who then resell to other player for a hefty markup. The price depends on markets, if there are more people selling than buying the price go down, and vice verca. You can avoid the 100% markup of the trader NPC by selling directly to another player. This is the same as selling a used car, you can get a better price if you find a buyer yourself than if you go to a used car dealer. You pay for the convenience.
During the BWE's the traders were often short on popular dyes or materials, because the price was so high that selling directly to players was worth the inconvenience for most people. This is what happens now with the sigil trader.
In the BWE's, the sigil trader was given a lot of sigils to start with, because one weekend is not enough to both win the HoH and do some serious GvG.
Anyway, just wait, economics dictate that the price will fall. The HoH is going to be won at a constant rate, while the amount of guilds being created is going to decrease.
Anyway, buy now, economics dictate that the price will raise. The existing guilds are going to continue to accumulate wealth by farming, so they will be able to afford increasingly inflated proces.
Economics is such a wonderful discipline
PS: Black dyes are not any rarer than other dyes, it is just that players are willing to pay more because 1) black goes with everything, and 2) black is more expensive in some other games, and thus more prestigious. The supply is the same as for all other dyes, but the demand is higher, thus the price. Some laws of economics are quite straigth forward.
PPS: Not all of us PvE'ers are whiners.