"What's Your Best Price?"

If they don't have any questions about the product itself, I find it hard to take them seriously. When their only concern is price, I assume they are fishing for things they can pick up cheap and turn quickly. That's O.K. because I too am fishing when I advertise.
Personally, I would suggest you follow Marco's advice. You've made the first move and paid $6.00 to do so. Now it's time for them to demonstrate some interest or commitment besides a mass produced "What's your best price?" I can imagine some guy sending out 250 of those a day.

I typically put the words "best pricers look elsewhere" in the body of my listing.

Sorry you think its 'moronic'. If you can't think of an acceptable response, then don't ever consider moving into sales.

The obvious answer...give your best price. What else would you expect to do? It's called negotiating. Sometimes I make an offer for an OBO item, and sometimes I ask for a best price. Depends on my level of interest in the item and the asking price. The combo of the two determines which way I go.

I will sometimes email a seller and ask for his/her best delivered price on an item to find out if they are flexible on the price and are willing to include shipping and or paypal. I hope I am not coming across as a moron. I seem to have good luck doing business on Audiogon and have never had anyone respond in an upset way.

I don't think asking "What's your best Price?" is any more "moronic" than asking "what's your best offer?".