I always post or respond with my ads "first come, first served." If you can't make it for three days, I'll say message me when you can make it and I'll let you know if it's still available. It's gained me some money on deals, as I'll tell future inquiries someone is interested and at what price. I also tend to post my machines for less than typical prices to make a quick sale, I don't care if I lose a few hundred on a game I don't care for. But nothing is locked until cash is on the glass. I won't tell a buyer it's a deal until they're picking up the machine.
Edit: I've put games on hold for people before and then they stop communicating. I don't know if it's just this hobby or life in general now with social media but communication lacks. I have a job where communication is key to safety and life. If it's a no, or you changed your mind, just say it.