You need to consider that some of the responses you received are from folks who try and pick up a nickel here and there. If you're a legit customer, you trade differently - you are in no real hurry (maybe you are) to liquidate a strategic position you built up, you can probably liquidate the same way you accumulated it.
If you're uncomfortable with all of this, simply call around to some of the bigger houses and see what they'll do for you.