Used Macbook Tips?

I want to buy a used Macbook for my son and need advice from someone who knows. It should only be used for writing, surfing the net and possibly Netflix. Doesn't need to be a high-end model. Better small and flat. The battery should not be too weak.
- which online shop can you recommend? If possible with a guarantee.
- which model up to around 500 euro?

Does it have to be a mac book? There are significantly more powerful devices for the price

For 500 euro you only get used ones and the batteries are usually pretty bad / worn out and the hardware is also relatively bad.

Would rather recommend a Windows laptop. You can get them new at the same price.

There are no new MacBooks up to 500 euro and I wouldn't buy used ones in this price range.

If it's only going to be used for writing, surfing, and Netflix, you could get an iPad with a Bluetooth keyboard for that money. Corresponding Microsoft Office-compatible Office apps are also available for iOS. And he would benefit from it longer than from a used MacBook with a possibly already run down battery.

Or just a Windows notebook.

A MacBook Air would be the first choice. You can play it safe at Apple's Refurbished Shop. For 500, - there won't be anything. First take a look at eBay to see how old those are who go away for 500.00. I think he would be better off with a new iPad. Keyboards are available for € 15 to € 500.

What for example?

The Lenovo thinkpad or ideapad range is quite good. Lots of choice and not too expensive either.