Why are the requisite skills, quality, required in a person to become a spare parts manager?

As a previous Automotives parts manager I would suggest you get ASE certified or something equivalent. I am not sure what the pay is in India for auto parts manager, but it should exceed when you are certified. This certification means you know what every part does and where it goes in the car as well as its functions and importance. 

It is truly one of the best jobs Ive ever had and wishing you much luck.  

https://www.ase.com/Home.aspx

Normally people come into part stores because they need a part. Not to much marketing or sales skills is needed, since they need you. The only thing I would suggest is to be informative and know your products. Different spark plugs for different makes and models and different performances, etc. 

For controlling spares inventory by meeting all necessary Work in progress vehicles.

Street smart, aggressive in sales /marketing, competitor watch and analysis, updates regarding current trends in automotive technology in after  markets spares/tires and understand customer mind set and understand their requirements. One has to work with Open mind , penetrate the market and try to increase the market share of the company products in spares/tires /engineering equipments.  

Being a manager is not just a job, its a career coz it can be added on your experience. But to maintain it, you should have proper skills and ability to your responsibility being a manager.