There is no point in adding "lowball offers will be ignored" to a listing other than being a bit snooty. Just ignore any offer the seller considers unacceptable.
As far as following an unwritten code, lowball offers are no different than unrealistic asking prices. 40% of new retail for a used item is in the realistic ball park. Some brands or models can be in demand and fetch more.
Once made an opening offer of 50% of new retail that the seller considered insulting. Asking price was 90% of what he claimed to have paid and $1K more than retail for the newest model. There is unrealistic behavior at both ends of the spectrum.