My MacBook is showing a flashing ?

This will help: https://support.apple.com/en-us/HT204323

..teesa77:

There are some who may sell second-hand their previously owned Mac

& then could have missed the "Before you sell" instructions to set it up.

If this happened to you, and the seller was responsible, they may fix it.

Could be an oversight. However there is a Possibility the Mac may still

have their Admin user account, inside. ~ If so, then this takes a turn..

• What to do before you sell, give away, or trade in your Mac - Apple Support

https://support.apple.com/en-us/HT201065

[The above article spells out: what a previous owner should've done.]

Usually good intent of ownership would be a new user account, with

temporary 'non-personalized' admin account. One that you'd be given

a password for; & had nothing personal inside, due to Erase & Install.

And if done right, you'd start the Mac & it would go through all the

'setup-steps' like a new mac. You'd personalize it, & it should work.

If an authentic reseller or auction (ebay, amazon, etc) had this, and

did not prepare it for correct legal sale, they'd need be contacted.

PS: proceeding post's link above, makes a reference to 'Question

Mark' issue; and details, should further attention need be focused

https://discussions.apple.com/thread/253402439

There are a few "how to reset Admin password" types of articles online.

Some vary in difficulty. Or unworkable, pending specific circumstances.

..All Macs have/had an Admin user account. The first setup-and-run does

require the original owner to go into, and set this up to access the Mac..

Here's some of those; with built-in variables.

(if these links say to download ~ avoid that step)

• How to Reset Your Admin Password on a Mac : HelloTech How

https://www.hellotech.com/guide/for/how-to-reset-admin-password-on-mac

• How to Reset a Mac's Administrator Account Password | lifewire

https://www.lifewire.com/reset-mac-administrator-accounts-password-2259977

Some or most of these suggest to use your AppleID. If you did not set up the Mac

that is an unlikely avenue to travel. ~ Yours won't work to reset another's Mac.

• Three Ways to Reset a Lost Admin Password in High Sierra - TidBITS (+ later macOS)

https://tidbits.com/2018/07/05/three-ways-to-reset-a-lost-admin-password-in-high-sierra/

• 4 Ways to Reset a Lost Admin Password on Mac OS - wikiHow

https://www.wikihow.com/Reset-a-Lost-Admin-Password-on-Mac-OS-X

Would be good with trusted long term friend; familiar these matters.

Without an admin password, a reset or PW renewal; best try & access.

If you've not previously set up this Mac, nor any Admin account; still

may be a way to change what's there, & trying to follow instructions.

This is where the trail may seem sketchy.

Good luck & happy computing!🌞