Yesterday I bit the bullet and upgraded my house because the home that I wanted was on sale, so I started thinking about housing and what to do with homes you no longer need.
1) Allow us to gift or transfer ownership to a friend/guild member, which would give that person complete control of that home moving forward. This can be a single one time use transfer, so homes can only be transferred one time only. This can be in the housing options when editing your home, so that it is easy to find and use. Something simple like: "Transfer deed and ownership of home to friend"
2) Allow us to sell the home back to the crown store for a percentage of crowns, maybe 50% or 40% back from original purchase, so those crowns can go towards upgrading to a bigger home. The amount can also be based on how long you owned the home, so if you owned it for 5-7 years maybe only 20%-25% back and 1-4 years 50%-30% back. This can be an option when you go to purchase another home that asks if you want to sell the existing home. A restriction can be that you must own the home for a minimum of a year and before selling you must have all of the items that you own removed from the house or it will not allow you to make the transaction.
3) In a perfect world ESO would have a housing agent in each capital city that allows the transaction. You could sell it or transfer through this agent.
Just a thought that I am sure has been mentioned, but it would be a nice option.