This isn't something I generally approve of, if players wish to sell property then there is a tool to do so. Limited periods of auction are likely fine, until there is a ruling otherwise, but I will be advising DMs to release these should they feel the auction is lasting too long.
One thing I am forbidding now though, is purchasing shops/quarter with the sole purpose of auctioning them off. I've started adding a lot more shops, and dropping the price so as to facilitate entry into the economy by new players.
If a shop is bought, left empty, and offered via the shop sign for auction it will be immediately released by one of the team.