Different traders show up at different levels. The Goblin and Black market are not permanent till you reach higher levels (iirc, after 90). The Travelling Tree dude comes and goes randomly. Also, some markets will not open to you until you are in a guild.
As for currencies, some of the markets use currencies that you have to work towards and save up for. The girl, Goblin and Black market are the only ones that take gold and diamonds.
If you are still having problems contact customer service in-game, and provide screen shots if needed.