Bought a banned Xbox One S, what parts can I use?

There’s not much you can do unfortunately. I believe that a device ban affects the motherboard. Apparently (per Microsoft) if your Xbox determines you are trying to circumvent the device ban by doing a factory reset or re-flashing the firmware your Xbox will brick itself.

Your only hope is to attempt to re-flash the firmware and pray that you slip by the tamper detection.

Install firmware on Xbox One S

Onto the parts question, I believe that the only part affected by a device ban is the motherboard. Everything else should be OK.

Console banned systems are worth more dead then alive since this (generally) isn’t something Microsoft will reverse. Use it as an offline/local multiplayer only system and treat it as an offline/LAN only system if you’re stuck with it.

Sometimes they do provide exceptions if you buy it not knowing (even if you knew and priced it right, I’d still try that line and be prepared for them to say no), but it’s generally only a one shot deal. If you were buyer #2 of the console in a banned state and someone else already did that, they won’t do it again. Because there are NO GUARANTEES it’ll be done, you should price it to the point you can eat it if it’s dead for good because you’re owner #3.

Back in the 360 days, the CPU identifier was used to block the console and you could use JTAG to get into the system to change it and remove the console ban before they patched the JTAG hack with the 2nd generation dashboard. While the Xbone likely has JTAG presence, it’s likely limited to factory testing today especially since they killed it early on the RROD 360 for what people actually used it for. There will be multiple parts paired to prevent CPU replacement, so you will basically need to replace the board and optical drive to reverse the ban - which may cost as much as a clean one.

Note: Before you attempt to take apart/sell your Xbox for parts you should try to contact eBay about the issue as mentioned by @mayer

If the Xbox one is similar to it's predecessor, the Xbox 360, then the only thing that is identifiable (some type of console id) would probably be stored on the NAND. The NAND flash is usually a little chip about 1-2 inches in black on the motherboard that stores the Xbox operating system and other important information. So all in all since the NAND is soldered to the motherboard you could probably salvage everything but the motherboard.

Also I would like to let you know that all this is theoretical and since no one other than Microsoft knows for sure how exactly the console is identified, this is just an educated guess by someone very familiar with RGH/JTAG modded Xbox 360s.