I've been wondering why some friends can't see some items in the shop that I can, and after doing a bit of looking into it it seems that there's a "Hidden list of known items", if you see an item on the Personal Shop tab then you'll be able to search it later.
An example is that a friend was looking for Chaser Arrow and was unable to find it, but when I gave them a minimum price to search within the Discs category, he found it and was right after that able to search for it.
The same happened just now with another friend and the Froid Gallows M basewear. I gave him a min price and subcategories to filter and he was able to find it.
This wasn't a problem in PSO2JP from what I'd played of it, not that can I recall anyways so I'm lead to believe that this is a bug. I'd like to bring it to the attention of the community and developers and if you experience this as well, please make sure to inform the developers so that they might be able to fix it.
For now, if you experience this your best workaround is to ask friends if they can find an item, and ask them how much it costs so you can find it more easily.