Can someone please explain how this works? I see people with hundreds of one item in their shops. I want to know how they do it.

Probably buying up any that are priced below theirs and sticking them into their own shop + those that they've accumulated over time. Possibly if it's not a shop that has hundreds of multiple different items it could be someone suddenly deciding to sell off a hoard.

Thanks

I have one of the bigger shops in Subeta. There didn't used to be a shop size limit (there isn't one now, but one has been threatened) so there wasn't any reason to not hoard stuff in my shop and price it. I figure if someone wants to pay my prices, then they want it more than I do. If nobody wants to, then I get to keep my beautiful hoard. I've been playing Subeta for a good while, so over time things sorta...accrete...even if I only buy a few at any one time.

This is also true of my Wardrobe. I seldom wear lots of stuff, but I have all the Tiers for that Achievement.