On topic, these humans just won't enter the depot. What the heck? The bridge is down, perfect pathing (I sent my military to them and back), yet they just...sit outside my fort? Huh?
They saw your distasteful, sarcastic mockery and sour rebuttal to another forumite's well-intentioned, if poorly planned, request and are standing speechless, with their jaws on the ground. Please, that was just rude. Avoiding spoiling any of the FUN for the new players is preferred. Answering questions cryptically and only mentioning adamantine and the various benefits it has, none of the drawbacks, is the best response, and if you simply can't NOT be an asshole, then just outright lie to them about it. I speak from experience when I tell you that having it spoiled ruins ALL the fun of finding adamantine for the first time. You aren't willing to praise the miners and dig in if you KNOW it will kill you! Even being tricked, you can laugh it off and prove your dwarfhood.
I have no idea what could be wrong with your human merchants. Sometimes the game just likes to spawn creatures with nary a brain cell in their skull. Try sealing the fortress to ensure you break their pathing, then open it up again. Not sure about that either, honestly. If not, try threatening them by sending the militia after them, then call them off before they actually start fighting. Might scare them out of whatever dead-brain zone they're stuck in.
Also, I can often get away with far less than 5-10% profit margins from the traders. a ~100 dorfbuck margin on a 1000-2000 dorfbuck purchase, sometimes better. It can fail though if you try to buy up an individual item that is very expensive like decorated, exceptionally made steel gear.
The first, best way to get away with it is to only bring a few goods at a time, and not too expensive ones. They're far, far less likely to refuse a poor trade if you don't wave your masterwork jewels and spears in their face and say you don't want to include them in the deal, so just don't show off too many of your goods at once. The AI is stupid like that, and if you act cheap the first year it'll take a while for them to start expecting more. Having a good broker helps, of course.