Personally, I wouldn't sell the server with the hard drives included, let alone with the OS and SQL Server installed on them. I also wouldn't use the hard drives in a server I bought from a third party seller if they were included by the seller. I'd ditch them and buy my own drives.
Why are you selling the server with the OS and SQL Server installed?
It has your license info (Windows and SQL server) and who knows what else on there as well.
I would scrub it clean with DBAN or similar so you know it is all deleted. If you feel nice, you could reinstall OS, etc. You may not be able to transfer the windows license, etc. (not sure, I'm a Free Software guy for a reason).
Additionally, if I were to buy ...
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