What should I do with a second hand SSD?

This is what I would do:

On the existing HDD/laptop, run showkey plus:
ShowKeyPlus

Record any product keys provided.

Install the SSD into the laptop. Boot the laptop from the Windows 10 installation USB flash drive or DVD. When you get the first windows to install Windows press Shift + F10 to open a command prompt. In the command prompt run:

diskpart
select disk 0
clean
exit
exit

That will erase all the partitions on the SSD. Select the custom install option in Windows setup. Select the unallocated space to install to and click next and let Windows do it's thing. Enter the product key provided by Showkey plus to activate Windows.

Done.

Thanks everyone.