It's a marketplace. Lowballs, haggling, sob stories that may or may not be true. It's all part of the game.
One thing I do though is I ALWAYS respond POLITELY, even if the offer is a really dumb lowball (like somewhere around 50% of asking price). Here's a template of my response:
"Hi _____. Thanks for the message.
Sorry, that offer is way too low/Sorry, I'm firm on my asking price.
Thanks again.
Earl"
It takes < 30 seconds.
I also do this for emails, even for sprawling one-line conversation email threads.
"Hi _____. Thanks for the message.
Sounds great.
Thanks again.
Earl"
It just sounds really polite.
Politeness is key! Not just here, but anytime you're dealing with people.
Sell sell sell.