I would not exchange money at Grand Bazaar unless it is a bank. Offering $ is fine, but price wise you will do better with the Turkish lira.
With haggling I start at 30% and go up. The prices there are not the best/lowest, I would go and tour it, an do my shopping elsewhere.
Beware of the fake "branded" products. For non branded and souvenirs it is fine.
Don't forget to buy lokum to bring as a gift. My fave.