make all the items less useless

Passives are not the way to do this.

Like museum sets, the way to handle these items are to exchange sets or make the items consumables.

Think about it...everyone already has these useless items...that's why they're useless...so if you just need one...it won't do anything at all.


Sets and consumables...that's how to do this.


For Otter's converter, I would say you're just bypassing a mechanic that already exists: selling to shops.

A better option would be to have items sell prices fluctuate by some formula accounting both a global flat rate (maybe +/-1k) and a % of the base sell price (maybe +/-10%) together. This would let the market become responsive to a random factor. investing in random items would stay relevant with the amount of cash used and nothing could take advantage of outlier items that are either cheap or expensive.

Granted, things that are like $200 in shop could have much larger turnovers, but you'd be relying on a large random factor so it'd be a large investment over a large amount of time.

EDIT:

Also, i would say that even that, while better, is unnecessary and overall a bad idea. It's better to have uses for these items. It wouldn't be hard to come up with uses if you just take time to think about it. In fact, i've already started it a few months ago.