When I worked in the TV repair business - briefly - a number of years ago there was a joke going around -
Invoice - To repairing TV £50.00
Customer - 'What was wrong with it ?'
Tech - 'Burnt out resistor'
Customer - 'How much do they cost ?'
Tech - 'About tuppence each'
Customer - '£50 for replacing a 2p resistor ! ! !'
Tech - 'Give me the bill back'
Invoice - To replacing resistor £ 0.02; To knowing which resistor £ 49.98
As Nick has pointed out, selling at the same price as buying in does not cover the overheads. Of course, if you were willing to work for free your boss could reduce his markup, but doubtless you too have bills to pay, mouths to feed and so on.
There is also the question of volume, high volume = low margin, and the reverse. Your local supermarket is probably working on an average markup of 5-10%, your local jeweler is probably working on an average markup of 200% or more.