An item mall for the free accounts where everything on sale can also be acquired through normal gameplay...
What exactly could be for sale?
Just storage options or level caps? I don't see how renting storage or level caps would work out well. Think of the whining when Johnny rents some storage with birthday money and then has to give it up later.
Keep in mind that cats, gear, etc are already f2p (via handouts) unless you drastically restrict trading options. And restricting/changing the trading system would have major consequences. One example, think about guild recruiting f2p ppl (basically everyone new) when you cannot give them anything.
Finally if you sell advancements (skill levels or any advancement "bought" with time investments now), you debase the value of the accomplishments for all existing players. Why level when you can buy your way to the top? This is p2w as Iala observed.