I have a feeling that this question has been answered before, and perhaps it’s even considered to be a matter of policy, courtesy, and/or common sense, but if so, I’m still in the dark.

If I have something that’s listed in a WTB ad, is it appropriate to ask how much the buyer is prepared to pay for the item, or is it my responsibility to respond with an asking price?

(I detest haggling and am not about to get into a back and forth discussion about a price.)

If you have something listed in a WTB ad, you are the buyer, so, I don't understand your question.

I think he means that there's a wanted to buy ad listed and he has the item that the person is looking for

If you have something for sale, you should state your asking price. It's your property.

In my opinion, it's rude to ask "How much will you give me?"

Such a question suggests that a seller is willing to take advantage of a buyer while, at the same time, concealing the price they would be willing to accept.

Do unto others...

I'll tell you this- I turned my back on a friend who behaved this way, back when we were young and building our collections. He did it to me and he did it to others. The difference is that he confided in me on his dealings and I saw what he was really made of, and I didn't want to have anything to do with someone who behaved that way.

Having said all that- sometimes, someone seeking an item will state upfront what they are willing to pay, in which case, there is no ethical dilemma, but, other than that, in matters of commerce, treat others as you wish to be treated.

Okay, thanks. I suspected I knew the answer, but didn't want to answer my own question.