Suggestion:
Have you tried waiting a couple days or tried searching for the Argonian yourself? He could be stuck in the map somewhere and knocking him unconscious might get him out. If he's not at the spot by well over 2-3 AM then you might be plain stuck. Oblivion was never known for being a perfectly stable game. However, you do not have to join the theives guild to sell stolen goods. You can always sell it to the Innkeeper at the Inn of Ill Omen and I think you can also sell stolen goods to the merchant in the Dark Brotherhood.
{ 2 comments… read them below or add one }
you can only talk to the guy at the waterfront about joining the thieves guild at midnight.
If he is near and not coming over simply push him there and speak with armand again