They get snapped up by speculators as soon as they are released or re-released. I'm not sure what to do about it, in a free market this is not forbidden. The moment you start reclaiming such properties by force, you create a system which is no longer free.
Actually, there is a solution since we're not in the physical world, but in virtual space. Virtual property can be created in any amount at almost no extra cost. How about this for any malls or high-rise residential estates: Create a bidding facility where participants can bid on a flat or shop in a location they desire. As soon as enough bids are in to fill a certain percentage of a floor (I guess somewhere from 50% but below 100% would be reasonable), add the floor to the building in-game and release the deeds to the bidders. The rest may then fill up before the next needs to be opened. In principle, an arbitrary number of floors could be added, only limited by platform restrictions. What this will create is a disincentive for speculation because the scarcity is removed. Possibly a number of inactive speculators will wake up then as well and release additional space. Also activate existing dead, merely decorative buildings for this and equip them with usable estates (Port Atlantis, Corinth, Ithaca, Troy to name just a few). Also, the rarity of shopkeepers is a problem as well as the absence of a directory where items for sale in private estates can be seen universe-wide like in auction. Doing business in Entropia should be easy and the entry threshold low, not high.
Tl;dr -- any upward pressure on price, add more. Because you can.