Might want to look at how you're wording your ads. At least in my city, it's normal to ask "will you take $x" if the wording isn't clear that it's a firm price, and those offers usually result in a completed sale.
I repeat that it’s final price and no-negotiation etc. This is stated in all communication. I have a good hourly rate so inconveniences quickly stop being worth my time. Lowballers don’t read. They’re professionally annoying for a living and there’s very little you can do about it if the marketplace allows it. Also Lowballers ask ridiculous prices, like 90% off an already 50% discount, they know what they’re doing and it must work for them often enough for them to do this seemingly full time.