Can I reuse a domain from one of my deleted sites?

Yes, you can reuse a domain from a deleted site.  However, you will first need to remove this domain from the deleted site so that it can be used again.  Please follow these steps:
1)  Go to Sites > Deleted Sites > [site name] > Settings > Domains tab.
2)  Enter a new domain in the "New Domain" field.  It can be anything you want as this site is a deleted site.  You may wish to simply add "deleted" (without the quotation marks) in front of the site's current domain.
3)  Click "Add New domain."
4)  If the domain you wish to use in your new/active site is set as the primary domain (indicated by a green check mark), you will need to highlight/click on one of the other domains in the "Domains for This Site" list and click the "Set as Primary Domain" button.
5)  Highlight/click on the domain you wish to use on your new/active site in the "Domains for This Site" list and then delete the domain by clicking the "X" button next to the "Domains for This Site" list.
6)  Save.
7)  Saving will reactivate the site.  You will need to go to Sites > [site name] > Settings > General Info tab and click the "Delete Site" button.  You can now use the domain on your new/active site.