It's a tough racket to be in these days, my business partner who's several years my senior (been in the business over double my time, and it's been 17 years for me this summer) regale me on times of gross profit, where a price sheet on parts ment something. Shops bought at jobber, sold to other shops at trade, customer paid retail. Everyone made a decent profit. Today, it's a constant battle, forcing us to do as much shopping as the customer does to get the best price and make a few bucks.
When you're busy, it's easy to have a "no outside parts" policy. There is no doubt that someone walking in with a truck full of parts is going to have some missmatched stuff, and most people on this site know that a bolt on part is rarely so these days.

It's also understandable for shops to have "chips on their shoulders" towards the internet parts sales and crate engine business, they errode the small shop's viability.
Trust me when I say that this industry is far from the gold mine it once was, for many of us have seen the market fall off sharply in the past 5 years or so, mainly in the stock engine rebuild category. In a nutshell, for my company, the shop used to support the parts department. Now, the opposite occurs.
I'm happy to work with a customer that brings in his own parts, but I need at least some of my time covered if I need to inspect/modify/exchange things. Often times it's tough to explain to the customer that he has the wrong springs or camshaft selection, or his bearings are the wrong size, etc. I'm working on a 598 BBC right now, the bulk of the parts brought in by the customer. In particular, BMF 405 heads, Manley valves of the required spec, springs, etc. After taking a good hour measuring, corresponding with BMF, etc., I had 2 options. Sell the customer new valves (big money) or order in Manton Lash Locks which raise the installed height .100". I chose the latter, now I hope once they arrive that they work as advertised.

Oh, and IIRC it was 3 hours worth of trial fitting and clearancing for the 7/16" pushrods this engine requires, plus the stress of risking breaking through the port wall while you're doing it, not nice. That part is finished, and no problems thankfully.:hurray: