You can make a fairly well informed decision and safely buy boots online if you are familiar with your foot size, last, instep height, and the features of the boot.
This "Go to a bootfitter" auto-response from everyone is all well and good, but it really is not a reality for some people.
For instance, I understand that I do well in a 100 last, 130 flex, medium-height instep, so I bought a pair of Nordica Ace 3 Star boots for $75 that were used 10 times. 10 TIMES. I dropped from footboards from my prior boots in and badda-bing, badda-boom, I had a $600 boot for $75 dollars that fit just dandy. I'll take them to a fitter to have the liner remolded when time allows, but my all in cost will never exceed $200.
Not every fitter is a good fitter. Not every user is a moron that can't make a well-informed decision. Not every kid can spend $500+ for new boots.