Bulk discounts for bulk quantities is fine. What collapses the value isn't that, but the incompetent sellers who come by later and aggressively undercut everything because they have no clue what they're doing. You can't really design a system to work around them without simply excluding them from the market board because their incompetence means they'll just ruin any market they come into contact with, regardless of what anyone else is doing. Market board slots are already very limited, so it's completely pointless to further restrict their usage. People who know what they're doing are already heavily incentivized to make efficient use of their slots, while people who don't know what they're doing will ruin everything they touch regardless of how it works.