I insist on messaging or email only, as I find it easier to deal with potential buyers like that. I find if I get a call I too go easy with that potential buyer as it’s in my DNA to be respectful & trusting that these people will stick to their word. With message/email, I can be more forthright by saying, “I’ll hold it till <specify date> and after that I will start fielding other offers”. That way it’s clear to them (in writing) & you don’t have to be a storage / standby facility.