Yes of course BUT you'll need to purchase the developer plan.
The developer plan lets you setup and re-use your developer license key for all client websites. If your subscription to updates & support runs out, all websites using your license key will continue to function but updates will be disabled.
No. Selling our themes on Fiverr is strictly forbidden. There are too many causes of license key theft results in genuine people being cheated out of their money. If you've bought a theme on Fiverr, report the user ASAP and ask for your money back.