It is easy to get annoyed with people making lowball offers, but understand that it isn't likely to change. Musicians as a group tend to have short attention spans and are always chasing the next shiny object. So once they've grown tired of a guitar, pedal, or whatever, they often just want it gone and will accept any offer just so they can get rid of the object.
Musicians also tend to be broke and in constant need of funds right now (rent, gas money, etc)
Both of these facts means that making lowball offers on musical gear often yields results and as such, the practice is never going to go away.
When I sell something (be it music gear or whatever) I decide my lowest dollar, put that in the ad and add "price is firm". Cuts back on the low ballers.