I know it’s tempting to either ignore lowballers or tell them to get lost but in my opinion, being polite is the best approach. It’s not all that uncommon to turn a lowball offer into a decent sale and even if it only happens every once in a while, it can still be worth it.
Some end users are genuinely ignorant when it comes to domain valuations, don’t take it personally.
It might be the first time they’ve ever heard that some domains are worth more than the regular registration cost.
You don’t have to always write them a personalized message, just put together a template and use it whenever it makes sense. That way, replying to lowball offers won’t take more than a minute or so, making the risk vs. reward ratio very attractive since wasting a minute of your time is the only risk.