I'm not an accountant & I suspect there are much better qualified forum members that have businesses who can chime in. John's first link is what I have referenced in the past. Registering for a business (BN) is the most basic, but is kind of limited. There may be vendors that will provide you goods on this basis alone that they otherwise wouldn't sell to a personal name, but I think its more a throwback to a prior era. Kind of like printing your own business cards at Staples, its basically a label. Some vendors won't sell to Joe Blow because they have a perception businesses will be bigger sales transaction (which may be true or false, but its their decision not yours). Others may be purely for liability reasons. I've encountered this myself. Some products the government has decided should not be off the shelf available to the public, only commercial.
I believe the next level, typical small business is when you also have a GST# registered/linked to the BN. There is probably some $ threshold when you require GST# registration. When you sell an item or service you add GST. When you buy inventory or tools you get GST credit. I know that when customs is involved, shipping requires this GST number. And I think paid consultants also require GST#. So yes, I think any above requires some kind of GST submission for tax purposes. The more fundamental issue is a business must have some reasonable expectation of profit (however they word it). If you buy a bunch of tools & materials for yourself & don't sell any product or services (no revenue) that becomes a loss. That's still not an issue in itself, but is basically asking CRA to have a looksee what's going on.
Getting back to McMaster, its entirely a company policy. They will sell to Joe Blow if he lives inside the USA, that's why Americans love the place. This used to be the case long time ago for Canucks, but as mentioned, they came to the conclusion that 20$ worth of bolts was costing them $50 in expense prepping the customs paperwork, so they ended it. But larger business accounts essentially no change. I know this because I spoke with them at that time. There could be some legacy accounts that they may still honor, that would be a good thing.
I wish they were accessible because they are fantastic 1-stop shopping, everything comes in 1 box. But I'm kind of over it now. I'll find the same items eventually because they don't make anything unique themselves. We also tried hard to order & drop ship to USA but they made it very difficult. I suspect they screen CC#, IP address... & deem this to be foreign based order. It looks like it goes through and then you get an email saying 'there was a problem with your order'. If anyone has any update to this I'm all ears but I think that's still the case. That then leaves options like a USA buddy who orders on your behalf, or a business re-shipper, which typically adds 15-20% to the transaction & more processing time.