I have offers turned off on eBay for this reason. The only thing you ever get is low ball offers. Yes, I know you can set a lowerbound limit.

Since I have offers disabled (or if someone wants to try to underbid your setpoint out of optimism and/or stupidity) that prompts the lowballers message instead. Usually with an insulting poorly spelled paragraph attached, or some sob story. Or both. But since they messaged you, that means you now have their user handle and can block them. So, goodbye.

Edit: In fact, speak of the devil. I had to punk exactly such a rando right now. They came at me with a 50% offer on an item I already have listed at roughly 50% below its selling price with their excuse being, "Well, I'm taking a risk trusting you that it works." Broheim, if it doesn't work eBay will force me to take it back, no matter what... The beauty of this, by the way, is that this precludes such idiots from interacting with your listings anymore and, I think, even being able to view them. So you'll never even see them again, unless they go out of their way to create another account. And they lose out on their chance to buy whatever your thing was at any price through their sheer greed and ineptitude.

I'm given to understand a sizable fraction of these dweebs try to make their living lowballing whoever they assume are desperate sellers of crap on eBay, and then turn around and list the same item right back for full price.