Actually I was going to suggest, the videos of Gary Vaynerchuk. So I'm glad that I can overcome all the curses and see and hear the true guidance it gives.
In fact, I'd suggest he do it, another GaryVee tactic, and go sell and flip things in the garage.
And this is the reason.
You learn SO MUCH to do this.
1 – You learn to negotiate face to face. Get someone from $ 10 to $ 5 (if possible) – you know what to push and pull.
2 – You learn product photography. How to make an image look good on social networks (and you should ALWAYS make your products look good).
3 – You learn sales writing. This is a big one Because to do this correctly, you must see it from your client's perspective. What would make them want to buy yours about someone who sells the same? What is your motivation? What do you REALLY want to achieve by buying this product? What are your complaints (or things that would NOT make them want to buy), etc.?
There is a great article by Neville Medhora where I learned to write sales copies that anyone, especially a 13 yo, can follow and understand.
4 – You learn about prices. And that includes mediating your margins and setting a monetary goal on how much you want to earn per day so that you reach your goal of how much you want to earn per month.
5 – You learn about customer service. Delivering packages to a person on time, setting up the shipment (or pickup) at your local post office, answering the questions in a timely manner, if someone has a complaint about how to handle the right way.
6 – You learn how to handle money. And you'll need your help with that, when it comes to automating savings, spending on things that really matter to you, etc.
There is a GREAT and fun book on the subject called "I will teach you how to be rich", which speaks of this which is practical and very readable (it is also in audio format). The title is stupid, but I promise you it's good.
All this from the sale in the garage and its launch on Ebay or Craigslist or whatever.
Build a GREAT FOUNDATION for any business venture you want to make when you grow up.
It's money in the bank at 13 years old.
And it is an easy way for him to get small gains that increase his confidence.
I hope this helps.