I'm new to rpgames and Oblivion, have joined the thieves guild, completed the quest concerning taxes, when I return to the fellow on the waterfront he doesn't offer me another quest, I have fenced over 10K worth in gold, not sure what else I should be doing to continue in the guild, any suggestions? thanks

you not only get the missions from only one place
sometimes you have to complaete the main story a little or continue to another city to obtain a new quest.

and by the way are yoiu sure you have completed the quest

Go read a walk-through.

If I remember correctly, you have to wait until someone approaches you and tells you the next place to go. Perhaps you're moving from city to city too quickly. I would suggest staying in the same place for a few game days. If that doesn't work, you might have to go to a previous saved game and complete the quest again.