I sympathize with you, I’m an old instrument horse trader and not a fan of getting stonewalled when negotiating. EVERYONE has their price, and it’s never what’s listed. Firm or not, I try to deal with a seller or buyer. Since I’m on both sides of the see-saw, I KNOW what negotiable, non-negotiable, firm, make an offer, best offer, and “this is my number” is. You’re always gonna encounter a thick headed person that’s not in the game on the same playing field as you. I just dealt with a seller, around 22 y/o that posted an Indonesian PBass for $33 or best offer. I contacted and said “consider it sold, I’ll take it”… 3 days later he said his best offer was $125, which was too high. I explained that Facebook Marketplace is not an auction site. Set a price and deal with it. He explained he’s young and needs as much $ as possible. It’s the reverse of best offer. In my eyes, best offer is LESS THAN asking. This young fella looked at it in reverse. Then he messaged me 3 days later to say that the prospective buyer didn’t show (normal for Facebook Marketplace) and asked if I would match the no-shows price. HAHA, aaaah no, now my offer was $20. He then told me I was an idiot, I said DITTO, and that was my dealing with that millennial. No worries, there’s another one coming along soon, like the city busses. Keep the faith, it’s all a matter of perspective.