I don't think it is unethical to raise prices if no sale has occurred.
It would be unethical to do it after a sale has been made
but before then the item belongs to the seller to do as they choose.
It may not be a great selling strategy, but I don't see what's unethical
about it, especially in a free market society
I guess it could be viewed as time-wasting, though?
For example, if a buyer is only interested in the item, because it is priced at a low level and then, as soon as you hear from them, you up the price (especially if you up the price to above that of your competitors); you've, essentially, wasted the time they took to look at the listing (possibly) and ask a question (definitely), haven't you?