Once the special levy is passed then that is a cost against the lots. If the owner sells before the special levy is voted on, then it’s up to their conveyancer to find this out from the search of strata committee minutes.
If the special levy is agreed on before they sell, then this would show up on the Section 184 sales information certificate that the Owners Corp is obliged to provide to prospective purchasers (You can find Section 184 here).
This is an incredibly detailed document that shows, among other things, future liabilities and unpaid levies due on the lot. It’s then up to the purchaser to decide whether or not to reduce the purchase price by that amount.
Either way, the current owners can’t escape their responsibilities. Personally, I would be encouraging them to sell up and go, ASAP. The “can’t pay – won’t pay” brigade just don’t get that responsibility goes hand in hand with the savings you get from living in strata and they really should be living in houses.