I don't know if these items have a cap. I have seen toad oil climb as high as 13k each. There is a person who has a mule on carby that logs in just to buy toad oil, then logs out... Every time the guild opens, very frustrating.
and as a guild, it has hours (9:00 - 23:00), and the prices fluctuate depending on supply/demand, and fame has no influence on prices. (You'll also get the dreaded "Please wait longer before making another purchase" message)
After buying ~100 iron sands at 436, the price went to 516 the next day.
This is just a sampling of the prices that this npc charges. Prices vary a bit depending on fame and supply and demand. Fame at the time I did this was:
"Oh, Rappa. I was just talkin' to me mateys about ya the other day." (It's enough to get the summoner quest, dunno much more than that)