He is one of the Thieves Guild's fences and will be the first fence if the Hero joins the guild. While he is one of the easiest to find when needing to sell, he only has 600. He will only fence goods with members of the guild.
His home is plain, there is little of value inside except a few books. Picking up anything is counted as theft.
If the Hero is a member of the Thieves Guild during the quest "Spies," and is friends with Ongar, he will point them towards Jearl's House saying there have been times he has seen and heard things inside when Jearl wasn't home, and he thinks they are living there.
Steal as much as possible and sell it to a local fence.
Find the Spies in Bruma and remove them.
Bruma: "Bruma is a perfect town for a Nord man of leisure. Bracing mountain air, good food and drink, and lively Nord fellowship."