A bootfitter will tell you which boot/size will fit your foot. A ski boot seller will let you choose which boot you want to try on, then ask you if you think it fits you
Of course there are fitters, and fitters. As a purchaser, it's hard to spot the difference.
Personally, I would be lost without my ski boot fitter, as he modifies all my footwear to take into account 2 lots of surgery and the subsequent loss of tissue and ongoing problems therein.
My Orthopaedic Consultant was pretty impressed with the footbed modification work done too.
Thank you
cem