@Akili Li:
Yeah, I was also wondering why it's like that at first, but then I reached the same conclusion
I'm sure it's absolutely possible to make it show the set as bought if you have all the items and not bought otherwise (in case you have at least one missing item). But it would be really misleading if you bought the whole set and sold one or two items - you still have to double check yourself what you already have, otherwise you may end up with additional set.
It could be made so the set is shown as bought if you actually have bought the set at Treasure Trove. But then you may accidentialy buy it again, if you've bought it previously from others and then got some FCs to spend at TT. So again, it's misleading.
Considering how troublesome it is, I think it was a really good idea to leave it like that. At least noone would make a mistake because of that - it's best to double check each set manually by yourself. Or just remember what you've actually done with all your FCs - though I'm bad at that