But if the phone doesn't turn on you can't authorize the removal. And if it turns on but a factory reset is enough to let you authorize the removal, you're back to square 1. Either way it's not feasible.
the phone is likely linked to an apple account. seems reasonable that if the components are approved for a specific phone and that phone is linked to an icloud account, that account could permit a swap?
That assumes that you managed to ask the previous owner to log into iCloud on another phone and "free" the previous one for repairs. I guess you could do that if he's buying a new iPhone from you, but still... it is similar to the Macs that are stuck on the previous owner's enterprise account.