I agree with Tvad. these questions don't usually come from serious buyers. Further, my best price, until I receive an offer to consider, is the one I post in the ad. Isn't that sort of implied? I recall one time however when the correspondence I had with an interested party before he asked "the question" had me comfortable he was sincere and ready to buy. I responded with a slight discount from my asking price and sold the item to him. Net is I think you can tell from the tone of the request whther it deserves a direct reply. The only inquiries I ignore are the ones that go like this:
hey - i'll give you $nn right now you pay shipping to my home. . . where nn is a ridiculous number below my asking price.