Bearings are measured in mm. All skateboard bearings are 8mm (inner race).
The only time it would matter is if you come across a few really old roller skate models (I don't remember which one) that use 7mm bearings (b/c 7mm axle), but I really doubt you would unless you're actively looking for it.
Yes you are right.
I forgot about the 7mm (skates) to 8mm (skateboard) bearings.
I recall some people bringing in their Bones Roller bearings once a long time ago that wouldn't fit the skateboard axles and it wasn't until they read the packet they realised they were for the older roller skates, not skateboards.
Funny thing is most modern roller skates seem like they have adopted the 8mm axles to fit with all skateboard bearings, as they often use Penny trucks and skateboard wheels anyway.
Not sure if or how many people with skates you see in your shop, but there were a lot more in the last few years getting skateboard parts to replace their own, mainly wheels and bearings, but as said some truck hangers and other things too.