I have bids 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.

Now that I have bids turned off, or if someone wants to underbid the setpoint, that makes the lowballers message me instead. Usually with an insulting poorly spelled paragraph attached, or some sob story. Or both. But since they messaged you, that means you can block them. So, goodbye.