Well let's see
1.The point was getting more xps and items for your characters for example the cloack of mirroring.
2.A small city.Try going to the city gates when you return to Athkatla.There will be a man who will ask for your help and mark Trademeet on your map
3.the underdark is kinda tough but rewarding.Don't worry about selling or resting there are inns even in the underdark
4.One new merchant is available at the adventurer's mart (comes with the bonus cd) and another at the copper coronet(comes with preorder edition).You can download Deirde (adv) with the ToB patch