I would add - as a buyer (and possibly someone who's hell to a sales guy) - I usually ask to be put in touch with a sales engineer, and if he can't handle me, I very nicely explain I'd like to talk to the developers to get a feel for their product - because the devil is often in the details.

EDIT: Also worth noting - there is value in recognizing a customer you can't handle (whether or not you should be able to isn't relevant) - and then following up later to find out how things worked out for them so you can find out if you screwed up an opportunity.