First off, Welcome to ECN! Personally, I would have thought something like a supply house would be your first port of call? Just one thing to be aware of, buying things like circuit breakers on-line opens you up to quite a bit of liability, whereas, if you get them from a store, you usually have some sort of "come-back", should the product be found to be faulty or even worse counterfeited.
I suppose it depends on who you buy them from. There are certainly some reputable companies selling name brand products (I have hod good luck with Dale Electric and there is always Amazon) but there is also a lot of value in supporting your local B&M store. If the local supplier goes out of business because you can beat his price online, where will you buy something you need today? I can see the value of buying "stock" items online in bulk but that is also what keeps your local supplier's lights on. It is a delicate balance between keeping your supplier healthy and remaining competitive yourself.
I have to agree with Greg 100% on supporting the local supply houses, be they of the corporate groups, or the slowly vanishing 'mom & pop" shops.
You can't call Amazon 'after hours' to get something you need in an emergency! But, most mom & pops and the corp houses offer 'emergency' services. (Perhaps for a price, but if you are a steady customer, you should be OK)
It depends upon suppliers and from where you buy. There are many companies where they provide you warranty and replacement policy if you get defected piece. If it's under warranty, then you can defiantly exchange your product. Or else complain customer care of that company that you got the defective product so they can also give you an alternate option for what to do next.
From your posted profile I see you are a welder, that's a good profession!
Basically, the point I was making is that the local supply houses (mom & pops) have been taking a beating from the big box stores, and on line sources.
To that, IMHO you have a local source that you can go to for materials, they will assist you in any issues with what you purchase there, and the mom & pops can provide a wealth of advice on materials.
The big box stores provide extended hours and are open seven days, (where allowed by law) which can be very helpful for something that you may need when the others are closed for the day. Advice from the big box can be sketchy sometimes.