If you have a Mechanic do the work for you buying your own parts saves you money.
Shops mark up the parts they sell at lest 15-20% and in some shops the mechanic gets a percentage of the part sales. Also, shops sometimes do not get a good deal on their parts; meaning the cheapest parts available as they buy from who they have an account with.
I will give NAPA as an example. The one place I work bought from them wholesale for resale on the repaired product. Yet often we got parts that cost more then I could have bought them for at the local Auto Parts store. But NAPA delivered the parts and they could buy them on credit and the Bookkeeper did not want to deal with and keep track of several different accounts.
The result was that the customer paid more for the job

Some negatives to buying your own parts.
If you bought the wrong part and the Mechanic tries to put it on. You would have to pay him/her for the time used to try to install the wrong part.
If something goes wrong later the Mechanic might blame the part you bought instead of the work he/she

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84 300D, 82 Volvo 244Gl Diesel