From my view point, if someone does advertise their cell number on an Advert it should be accepted that they respond within a "reasonable time". A "reasonable time" is different for most people, some think a few hours others a few days. This also means that buyers should not be spamming a seller and vice versa. It all comes down to everyone has a life and has things to do.
While "lowballers" are irritating, it is something all sellers will encounter at some point (generalising of course). While we all have a different manner that is deemed as correct. Most times a simple "No thank you" or "This item is not negotiable" or even a simple "The lowest I will do is "RXXXX,XX". Also one thing to bear in mind for lowballing, What you deem an item worth others might not deem it to be worth the same and this works for most things.
As per mentioned earlier with telemarketers etc. Bring the car dealer story up. If someone does request a call back or an e-mail back, it should be common courtesy to simply respond back to an e-mail or to simply cancel the callback (If possible of course). Unfortunately many people simply don't work this way and believe the world revolves around them.
As Matt mentioned before a "Speedy responder" badge could work, however how would this be moderated, would a prompt come up after a transaction asking to rate various categories deemed as important? Respectfulness, honesty etc or whatever else is decided upon? How admin heavy would something like this be? What happens when people start faking stats and reviews like many popular online marketplaces?
And yes lowballers and disrespectful buyers are really annoying but "Cost of business" I suppose. It is a free market after all.
