what are the advantages and disadvantages of having a alignment of evil?Good?Neutral? besides special abilities?

If you're evil, you can finish quests in a different way: Possible Spoilers
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For Trademeet, you can kill the druids with a poison obtained by an evil man who approaches you. In Windspear Hills, you can side with Firkraag and kill Garren Windspear.

it also affects base reputation
lawful good = 12
Chaotic evil = 8
the rest are somewhere in between

You can take your party different ways too. If playing an evil character then naturally you would align yourself with other evil characters (if truly roleplaying) and good with good. I have played once through with a good character and a good party and now am trying with an evil character and evil party.

How do you get to trademeet?

Go to the city gates and talk to Flydian. He'll give u directions. Go outside the town to Trademeet.