Why so many Flaky Buyers?

I have had multiple experiences of buyers who respond to ads for items I have for sale who do the following:

Say they want to "move on this sale", "I'm interested in buying it", and then never respond when I email them back, asking for confirmation of the deal.

I have had numerous 'buyers' who ask "Is is still available?" and then never email back.

Buyers who say in their initial email that they are very interested in the piece, then later say they are not sure what they want to do.

Buyers who insist on lowball offers on a piece advertised as firm, trying to shave hundreds of dollars off, and get free shipping as well.

I'm honest, straightforward, amiable & accommodating, so I see no reason for this.

I'm really sick of this, and wonder if this is a common experience others have.