I bought a used Nexus 5. How to return to a completely brand-new state software-wise?

I've bought a used Nexus 5. For some reason, while giving me everything including the actual receipt from 2014, the previous owner left all the software, including TWRP and BusyBox, installed. How do I get rid of any traces of the previous owner's activity? I want to return to everything stock, including the recovery etc.

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