The shop search lists a particular number of the cheapest of an item. I won't say the exact number, because it gets adjusted based on server strain. If your item happens to be more expensive than that, then it won't show on the list.

You do have a shop search in your shop that you can use while questing along with the user shop search to make sure you don't miss anything! Or the account search, which will let you know if it's in a shop or your vault (because I'm very guilty of just shoving everything in my vault).

Okay, I'm locking this as it's been confirmed that it's working as intended. Bug reports are not the place for discussing these opinions - please feel free to make a topic in Suggestions or in Site Feedback for that discussion. Thanks.