Whats up with Ongar the World-Weary?

I am currently trying to join the thieves guild and am on the 'Independent Thievery' mission, where I have been instructed to sell about 50 gold worth of items to Ongar the fence, in order to get commissions for jobs. I just so happen to have a stolen flawless diamond worth 100 (that I could probably get him to buy for atleast half) but even when the barter option is available I can't seem to sell the stolen diamond to him. I have tried various times of day, yet he still doesn't seem to want to buy it. Am I missing something here?