trying to buy a house in riften. i've heard you have to do several quests with the locals in riften, especially the skooma quest you get from the argonian at the docks. well i've done nearly every quest in the town and i've done the radiant bandit bounty quests from the steward countless times and i still don't get the option to buy a house.

i've completed both thieves guild and dark brotherhood, does that have something to do with it?

The Jarl requires buying a house before he makes you his Thane, so ask him (or her). Maybe you should help the locals as well.