Not sure what constitutes bulk. Is it greater than 1? Is it equal to 999? If it's non-single, you'll have the same problem with players selling stacks of 2 and greater.
It sounds like additional search options would be more useful rather than add single/bulk. If you could also filter by stack size and price the same way you filter level, this may get you what you're looking for? If that's not it, it would probably help if you told me your exact use case for what you were trying to do here so I don't have to guess.
-ALS
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