When a property sells can I remove it to make room for another website?

If you have a single property websites subscription plan you can delete a property when it sells and you'll have room for another website. You can have as many websites as your subscription plan allows.

If you launched the website using a credit you can't re-use the time remaining on the credit for a different website. i.e. Credits aren't transferable between properties. The one exception is the first 30 days after the credit is applied, during which you can still change the property's address. See Can I change the address on a property website after it's created?