As a frequent business user of McMaster I find it serves a very particular purpose - everything on there works and does not break. However, their markup is massive. I once placed an order large enough that they redirected me to their supplier and found that they had over 100% markup on their orders. You're paying this markup so that McMaster verifies the quality of what you're buying. This makes a lot of sense for a business - you're not wasting employee time on reading Amazon reviews. I doubt it makes much sense for most consumers.
I'm continually disappointed with the quality of fasteners bought from Amazon. What you get from McMaster is much better. The problem, I guess is that Amazon is too low quality and McMaster is too high quality.
I also don't think they mark everything up 100%. I have built a lot of stuff out of T-slot extrusion ("80/20") and their price seems exactly the same as buying from 80/20 directly. I guess there is probably some Aliexpress vendor cheaper than the brand name, though.
