Expanding on my previous idea, it would be nice if these mystery box sellers allowed you to choose a number of species that you were interested in and could house, that would constrain the range of what could be in the box.
For example, let's suppose a buyer couldn't decide whether they wanted weather loaches, bucktooth tetras, pink tailed chalceus, or pictus catfish. They could select those species as the species the box could contain, and be pleasantly surprised with whatever 1 species is in the box when it arrives instead of having something unwanted and unsuitable.
So, instead of it being a "full blown mystery box" where the buyer hasn't got a scooby doo what's in it, and they could be getting any one of 1000's of species, you want to turn it into a "unmysterious mystery box", where you've narrowed it down from 1000's, to just a handful?
Granted, it's a more sensible approach to this stupid idea but it sort of takes the huge surprise element out if it. I prefer the scenario where the guy with the 10g tank is hoping his mystery box contains a couple of snails, but receives a baby arapaima, lol.