I'm going to message someone on fb marketplace at 11:30pm.
Why are you going to message them so late?
I want to sell some of my unwanted shit.
It's generally a better idea to try to contact someone when it's not so late at night, it's probably fine since it's just a message on Facebook. It's unlikely that the message would wake them up. Just don't get annoyed if they don't respond right away since they are probably sleeping.
I'm gonna pester them until they respond.
I would not do that. It will just annoy and frustrate them when they do finally talk to you. It will make them not want to do business with you and may lead them to telling other buyers about your behavior and giving you a bad reputation.