Is a locked Macbook Air M2 eligible for spare parts?

The MLB is worthless, but you may be able to utilize the other parts to some degree. The Lid Angle Sensor is paired with the Logic Board like the TouchID button. The TouchID button will work as a power on/off switch, but not with fingerprint recognition if moved to another MLB.

Otherwise the parts may be usable. It is hard to say for sure because Apple doesn't really provide any details. There is a chance some functionality may be reduced, but I cannot say for sure. For example, will Apple Pay & Apple Wallet complain about an insecure system because the parts are not all properly paired? Will HDCP protected HD content from streaming services or BluRay discs complain because the parts are not properly paired?

You may want to check iFixIt as their site may have some information or someone there may have a better idea. I know their repair guides for iPhones usually mention what functionality will no longer work when using an unpaired part or a third party part. I don't recall seeing anything regarding the T2 Intel Macs or the M-Series Macs.

As long as you clearly mention the locked nature of the laptop, then you should be clear for selling it for parts....at least your conscience is clear for being up front & honest about the condition of the laptop. I would even add a note saying that Apple will not unlock the laptop to help make it clear about its condition.

It sounds like the M2 MacBook Air is Activation Locked. It's very suspicious that the customer who returned that notebook did not unlock it using the proper Apple ID and password, and suddenly has stopped all communication with the store. If it was my M2 MacBook Air, I would definitely be in communication with the store, wanting to get that notebook fixed. If it was my M2 MacBook Air, not one that, you know, "fell off the back of a truck."

The motherboard of that MacBook Air and all of the stuff soldered to it – including the RAM and SSD chips – will definitely be tied to the Activation Lock. I don't know what the deal would be with respect to the battery, and the trackpad, and screen, and the keyboard.

However, I will call your attention to this:

Apple Newsroom – Apple to expand repair options with support for used genuine parts

"New parts calibration process will begin this fall; Activation Lock to limit use of parts from lost or stolen devices"

Apple intends to make stolen iPhone parts as useless as stolen iPhones. You can draw your own conclusions about whether they might also have similar goals with recent Macs.

So how is the MacBook Air stuck exactly?

If the customer lied about not being able to log in to iCloud , then what is the truth?

Was he able to log into iCloud?

If you have the original Invoice from Apple or an authorized Apple reseller, Apple should be willing to unlock it for you.

If you don't, then there's nothing that can be done. The Activation lock is a firmware lock and cannot be removed. Replacing any parts which would take considerable soldering skills will not remove the lock.

Your best bet at this point is to sell it for parts on ebay. With the specification that it is Activation locked so the logic board itself is unusable.