It is allowed. Yoshi made a statement saying it was, though, selling for huge profit is another story, that can and will get you into trouble if enough players report you.

However, if you own a plot privately, and want to move worlds, you can now remove your building permit (don't give up the plot!) and on the world transfer page, it will tell you that, you will get the full price of the plot + 3,000,000 gil to cover any non-removable items, even if you had none this works. When you login, you will automatically receive your plot cost + 3M Gil.

This doesn't work if you have an FC with a house.