When doing Saggitarius quests, one of the items he requests is the Plain Sapphire Circlet. When I go to purchase it from the site shop, I get an error stating 'The value cannot be less than 0', even though the quantity showing is 1, and it won't let me purchase it. I can, however purchase it from one of the user shops with no issue. This has happened on a few occasions.

I can't edit my original post without getting an error, so ETA this is also happening with Dusk Spirit, from the NPC shop.

That's because of the restocking limits on Kumos. When it says that, it just means you've hit the limit for the day. The wording makes no sense at all, but if you look above the input box, you should see something like this:

Daily Limit: 1 per day (1 purchased today)