It's actually the sense motive skill that is required. >9. If you have the golden orb, you can use it to cast discern lies for the talker. Or if evil + evil party you get an option to kill her just because you are evil. I believe that is a no. As I mentioned, the dialog was a bit tricky. I assume you did not attack the tower and you need this dialog option reply by Lareth (I looked it up in the dialog files to be exact): after which, you'll ask: And then he tells you that he will be a vendor - which occurs after some time interval. If you didn't have this particular conversation, he won't be a vendor. So...iirc if you ask for gold, the conversation does not include this sequence and he is not a vendor.