My understanding is that you can buy a minimum of $500 worth and also allowed to retain a minimum of $500 worth when selling part of the holdings. For example if one wants to sell CBA and wants to keep a small number of units -to avoid filling registry forms when buying again- the minimal number of units to retain must be worth $500. Say, total holding CBA of 1000units ----- selling 880 and retaining just 20 units @ current value of $30 = $600 (minimum value required $500). Later on these 20 can be sold at any price, but the buyer's order must be of a minimum value of $500.
