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1979 300D 040 Black on Black - 1985 300D Maaco job (sadly sprayed over 199 Black Pearl Metallic) on Palamino

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The original poster framed the question in a neutral manner ( thanks ) as opposed to other threads where it starts off " I want free work."

Having been in and around the auto repair industry for 40 years ( 10 owning a shop ) Bought parts are generally not good practice, years ago a "homeless" ( RE regular shopless) person wanted a bought part installed on their car, right _NOW_. I sent them on their way stating that I don't install bought parts. . . .then I proceeded to warn other shops in my area that he was looking. Not one shop took the job and the "customer" was fuming by the time he hit the 5 th shop.

Different guy wanted bought parts installed made a fuss but relented and I bought the parts. His wife called to tell me that she had advised him not to buy parts and didn't have an issue with my decision. . . And, wife was a caterer who specifically stated she fully understood the bring food to a restaurant analogy.

I would let regular customers source parts for odd cars ( this was pre internet ) as chasing them would cost me more than I could make when marked up. However, their regular cars got parts purchased by me.

My standard reply to bought parts was:

" I don't install bought parts and here is why.
First reason, if the part is wrong it ties up my shop.
Second reason, if the part fails, who gets the blame? Usually the mechanic does.
Third reason, I make a mark up on parts, I am entitled to make a living.
You will find that most better shops don't install bought parts."

While we are on the subject, my reply to someone that wanted the car fixed as cheaply as possible was.

Me: So you want me to just patch it up?

Them: Yes. . .

Me: . . . And then I get to fix it the right way when it breaks. . .for free. . .

Overall, if a business isn't having 20% people walk away due to price, you are not charging enough. ( unless your customer base is closed to new customers and there is a waiting list )

Three ASE certified mechanics I deal with do not mind the customer bringing their own parts.

Universally, the part is not warranted.

The caveat is that I feel comfortable doing this because I do have some sense of auto repair/mechanics and like to do much of my own work.

I also go the extra mile in finding the better parts. That alone can be time consuming. (so that is another variable, if you don't have the time to spend researching ever changing part quality reports etc,then let the mechanic deal with it)

I have had an instance when the certified mechanic provided their own part (Bosch glow plugs) and all that was available to his shop turned out to be the 'made in India' stuff.

They failed and blew out my GP relay.
I independently bought good European beru ones (and a new relay) and replaced them myself. That action was after I amicably phoned him. It was faster and more convenient for me to do this than travel the distance and leave the car at the shop (towed).

I returned the bad parts to him for a parts refund. No hassle from either side and I did not rag about the relay, or even mention it.

So with that anecdote, I might venture to say that even ASE certified mechanics are often vulnerable to whatever suppliers have available. More so true for vintage cars.
Yes many mechanics know the ropes and try to get good quality parts in order to avoid returns etc. That only would make sense.

I still like to bring my own parts and the mechanics have no problem with that.

Caveat, in my neck of the woods, labor is very expensive. Alot more than in other parts of the country. So, hypothetically, If I had access to a close by border town with less costy labor, then I would just let the shop provide everything.

Another caveat, some mechanics charge by the book whether it takes the 2 hours by the book or a very real 45 minutes. So I try to educate myself on what to expect and measure what a shop might be quoting, variables considered.

So lots of caveats. The more I personally save, the more I am willing to spend on having a mechanic do labor or whatever in order to preserve my cars.

I feel its a personal choice and these prior posted responses have highlighted the 'give and take' aspects quite nicely.

So good luck OP.

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1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
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1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD

I have handled this brought parts problem like this:

I bring my own parts to the mechanic and tell him that he can charge me an upcharge of 10% of what I paid for the part, because I understand that parts markup keeps his shop open.

That way I get the part I want instead of the part he wants, and I'm acknowledging that he has costs.

This may not work the the OP, who is looking short term it sounds like. A good mechanic is gold-- you dont mess with a good mechanic.