"What's Your Best Price?"

If you dont want the hassle dont offer OBO and say the price is firm, you cant fault a buyer for asking when your the one who gives an impression of negociation. Once again this is a thread that has popped up before.

"What's your best offer" usually separates the buyers from the browsers.

I agree too.

I agree with Sgood51-

I know what I'd like to get, but I also know what I'll take to sell it "right now" and that price may be different at different times. If I've just listed the item, my best price is going to be close to what I've listed it at. If the item has been for sale for a couple of weeks, my best price will be a little lower.

It all depends on how badly I want to sell the item at that moment.

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.