How do I sell my house?

There's an option to sell the house when you move out: How to Move to a New Home in The Sims 3

But, it sounds like its too late for that method as you've already moved out & bought a new house. Your option now, I believe, is to use your Sim's phone and/or computer to manage real estate.

Select the sim's smartphone or pc.
Select real estate & travel services
Select check real estate
Select private lots tab
Look for your old residential property that you want to unload & sell it.