I think A-net implemented new code into the traders.
Basically they inverted the way a trader works in that when you sell items to him the price doesn't go down, but it goes up.
My basis for this theory is that in the past when you sold sigils to the trader the price would go down roughly, and in large quanitities the price would drop dramatically. This is how sigils got to around 30k.
Well I recently sold 10 sigils to the trader and the price went UP by 3k, and its still going up to the point where they are now around 100k.
This could also be the reason silver dye keeps rising because the price was around 1k so ppl start selling silver dye to the trader and the price goes up and up and up.
And I have no idea what happened with black dye but yesterday morning someone brought to my attention that it was 1.3k. I bought 73 of them and went back to storage to get another 100 plat. At this point he was completely sold out of them from everyone buying them.