Believe it or not the VERY SAME THINGS happen on Ebay and Etsy. It is what it is, and people are what they are. I can't tell you how many questions I have asked, extra photos I have provided, and then negotitate a price to have them flake or give a sob story. And I never list OBO, they ALWAYS hit me with half the value or less in their offer. And I ALWAYS list higher than what I hope to get. On OfferUp I simple respond "thank you but I will have to decline" or "Thank you for your offer, I will accept $...." If they were serious they will continue the conversation, otherwise they just drop off.
My first actual flake. We negotiated a price, she agreed, and drove all the way to my store. She stood there and let me take it down from it's display, SHE carried it to the register, then said she needed to run out to her car. She took off! I messaged her and she said her son needed a potty break and that she decided she would look for something larger. She thanked me for my hospitality and wished me Happy Holidays. A flake, but a nice flake LOL
When my price is firm, and the potential buyer offers less, I respond with a higher price. I've actually sold things this way, as I post items way below market value to move them.