Should I buy a used iPad or M5 without an invoice?

Be very, very careful about buying a second-hand Apple device - as you could be buying something that is Activation Locked and cannot be used. This is a principle Apple security feature that is designed to prevent use or resale of stolen devices - and the Activation Lock can only be removed or disabled by the owner whose AppleID is “locked” to the device in question. Neither Apple nor anyone else can disable the lock if is set.

Legitimate authorised retailers are the only truly “safe” sources from which to purchase a pre-owned (and/or reconditioned) Apple device - from whom you should have confidence in purchasing.

The All Too Common SAD Reality of Buying … - Apple Community

Here is what Apple suggests when purchasing a previously used device:

Check for Activation Lock before you buy a device from someone else

Before you buy an iPhone, iPad, or iPod touch, from someone other than Apple or an authorized Apple reseller, make sure that the device is erased and no longer linked to the previous owner’s account. Follow these steps to check if the device is ready to use: 

1. Turn on and unlock the device. 
2. If the passcode lock screen or the Home screen appears, the device hasn't been erased. Ask the seller to completely erase the device by going to Settings > General > Reset > Erase All Content and Settings. Don't take ownership of any used iPhone, iPad, or iPod touch until it's been erased.
3. Begin the device setup process.
4. If you're asked for the previous owner’s Apple ID and password, the device is still linked to their account. Hand the device back to the seller and ask them to enter their password. If the previous owner isn't present, contact them as soon as possible and ask them to use a browser to remove the device from their account. Don't take ownership of any used iPhone, iPad, or iPod touch until it has been removed from the previous owner’s account.

You will know that a device is ready for you to use when you're asked to "Set up your iPhone," “Set up your iPad," or “Set up your iPod” the first time you turn it on.

LotusPilot provides the basics. Also note that should you ever have problems with your account, lose access to it and need to use a different account, you will need to apply to Apple to have the device reset. In that case they won't do it without the invoice. --> https://al-support.apple.com/#/kbase and here --> https://support.apple.com/HT202711

If you want to buy a preowned device refer to this article --> https://support.apple.com/104999

as well as --> https://discussions.apple.com/docs/DOC-250001038

Don’t buy.