When you're meeting people for whom you'd have to adjust your plans in order to connect – such as when someone is going to pick up something you're selling on Craig's List – be sure to ask when they will be there.
A half hour or even an hour window is better than sitting around for hours and then finding out they got lost or distracted or their friend who's giving them a ride had to do something else first, etc.
It's tedious to be in limbo, so do what you can to avoid it by knowing when you're out of the window of obligation.
Should you still find yourself in this state anyhow, figure out what things you might have been doing here instead and do that while you wait.
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