I wouldn’t touch that sale with a proverbial barge pole, not the least of which is because of Activation Lock.
Online markets are rife with offers of lost or stolen property.
Which will be useless once purchased, when Activation Lock is set.
And there is no reliable remote means to verify Activation Lock is unlocked.
Details of: Activation Lock for Mac - Apple Support
Trusted sellers only. Or if you’re willing to speculate, sellers that you can meet and check before payment.
One unfortunately common scam with used iPhone sales involves blacklisting it after the sale, too.