Who's responsible for burnt cart?" => cave and letter, back to Marcus => the one you busted, cause you had the sewer letter first IIRC

What's all this about the black market?" => the quest is different, maybe "black market depot"? That's the part with Tingalf, sewers, Pedro, letter


Methinks someone is selling goods without a license." => one of the speech options to Robin for solving that http://www.kiya1.de/images/Div_Robin1m.jpg.html => look at the blue crosses in this link and compare them to your own conversation log with Robin. If you had them, you can talk to Trevor and this part is solved.

Sorry, but I have a list of the quest log => check this, please.

http://www.kiya1.de/images/Divexp_Marcus5logm.jpg.html
http://www.kiya1.de/images/Divexp_Marcus4logm.jpg.html

the only thing you can do is talk to Trevor and see if something happens. All other chars are obviously through.
Kiya