There is a difference buying and selling parts for resale versus using them in repairs. And there are differences in the type of labor.
In California, labor for repairs is not taxable, but labor for building or fabricating new items is taxable.
https://www.boe.ca.gov/formspubs/pub108/
Parts used in repairs, if they are more than 10% of the total cost of the repair, "the repairman is the retailer and tax applies to the fair retail selling price of the property."
If the cost is less than 10% " the repairman is the consumer of the property, and tax applies to the sale of the property to him."
http://www.boe.ca.gov/lawguides/business/current/btlg/vol1/sutr/1546.html
If you are buying for resale then a different set of rules apply.
There are many members here who buy hardware or software at retail, from Amazon, NewEgg or wherever and resell to their customers. Do you charge tax for those?