The wording's always been very ambiguous in regards to receiving  the 6 digit code and from some of the questions on the boards,  it seems that some buyers  haven't received the code if they intended to pay  eg: cash

but of course  it could be that they haven't used the app.

In one part, it says

"The seller will let you know which payment methods they accept. In addition to paying via eBay checkout, you can sometimes pay for local pickup items by internet banking, cheque or money order – either beforehand or when you pick up your item. Sellers may also accept cash for local pickup purchases"

followed by

"After you've completed your purchase, we'll send you an email with a local pickup code (a QR code and a 6-digit code). You can either take the email with you or show the seller the code in your eBay app when you collect your item"

So does completed mean they've gone through checkout and paid by a method covered by eBay's money back guarantee?

Or not ?

Good idea for the OP to get the invoice signed but really no reason to  if the buyer paid by cash, 

as the MBG doesn't apply.