I can't answer the question - but I'm sure it depends in part on the brand of skate, just like normal shoes. (And, for that matter, you might fit one brand's narrow but another brand's wide. Even the length of a size 8 varies substantially by brand, as it does for normal shoes. And you might need to know whether a size 8 is a child size 8 or an adult size 8.) In addition, it was potentially affected by possible customizations, such as a mixed width or custom fit orders.
Because I've had a lot of trouble with skate fit, I would personally hesitate to buy by mail - especially from a non-expert.
But it's your choice.
If you could talk the seller into temporally removing the insole, scanning it (a lot of printers now have built in scanners) with a ruler in the same scan, and sending you the scanned image, you could then print it, zooming it up until it matches the ruler scale, and see how it fits around your foot. Do for both feet, of course.
That isn't the whole story, as feet are three dimensional. But it is a start.