These days I buy almost everything online, but there are exceptions:
Shoes - Not unless they are replacements for ones I already have.
Clothing - See shoes, above.
Groceries - Never, because I’m real picky.
Speakers - See Groceries, above.
OK, I have to confess I recently bought a pair of KEF speakers online without ever auditioning them. The local dealer told me he did not have a set available for audition, and could not get me a pair before the KEF 20% off sale ended. I only did so because the company I bought them from said I could return them if I did not like them. Maybe they thought no one would be motivated enough to drag a couple of 82 lb boxes to FedEx. Clearly they don’t know me. 
Luckily I’m happy with the purchase.
So what does this have to do with Ethernet Cables and Routers/switches? Nothing, but I’d be happy to buy them online as I’ve tried to listen to the difference they make and could not find any: