I doubt a mechanic would accept your parts. You could buy the best parts in the world and unless u know the guy or something, they probably would not take the parts.
First of all the want to put a mark up on the parts to make money, and second if somethin went wrong with the part and it was the part u gave him, they would blame the mishap on the part rather then mechanic error. I would just take it to the shop and have them buy the part and repair it.
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Black 4 door 1990 325i
15"x7.5 Ronal LS-wrapped in 205/50-15 Bridgestone Potenza G009