Quests won't really net you money as most quest payouts are hilariously minimal (500 gold? HAH!). The real money comes from looting during those quests and then selling the stuff afterwards.
The problem with doing the Thieves Guild fence is that it's very time consuming to get the fence up to 4k per transaction, and even when she hits 4k, it's still only 4k lol. That's a lot of waiting 48 hours for her money to reset.
If you do the Riverwood guy like I said, you have to take the perk that allows you to sell any item to any merchant anyway in order to get the "Invest" perk, so not only will he have 10k+ in gold to barter with but you'll be able to sell him literally anything from your inventory. No item managing, no nonsense, and you can get 10k in gold before having to worry about the 48 hour wait period.