I believe you can sell for any price. They can hold any amount from selling but you can only pre-load them with a certain amount of gold. If you want to get gold off them just use the vendor's menu, I think it's "Retrieve gold." (It's towards the bottom of the menu.) The gold will be deposited into your bank box.
*Edit - Ok, ignore the first part of my post. I think I remember reading that they changed this.