On 2011-11-14, Larry Jaques wrote:
You're right to do that. Say "Buy the item and then we can talk." Or,
when they come up to you, set your timer watch for ten minutes and
when it beeps (or if they ask) tell them that you chat for ten but
charge $50/hr (w/ 1 hour minimum) for consulting time. It takes 3
seconds to set my Casio diver's watch to countdown mode. I use it to
steep my tea most often. 
I'm about ready to start charging for estimates, myself. People who
have no timeline feel that others don't, either, and will readily
waste a couple hours of your time if given half a chance.
Maybe you should start charging for estimates that take longer than,
say, 20 minutes.
i