My advice is to ignore them, or as I do counter offer the listed price or a tad lower if you are willing to discount. 

If you like you can add those buyers to your block list, as some sellers do.

eBay is a marketplace with all kinds of buyers and sellers, including those that message with offers far below listed price, you don't have to sell to them if you do not want to, so nothing as far as I can tell, to be concerned about.