This should be part of your initial contract, an exit clause. What happens if you get hit by a bus? The client would not have access to transfer the site to their own systems or host, right?
- Any purchases for physical assets (i.e. hardware, peripherals, physical media) can be purchased by you or the client, and given upon payment.
- Any online license purchases should be sent to a mailbox the client controls. They should know how to access it, and that you have access to it. It can be as simple as a GMail account created for this particular purpose, or a new alias on their email server. Guide the customer to the purchase screen, even if you need to use screenshots to make sure they order the right thing, but make sure THEIR information is entered.
- When building apps, you need to make sure they are transferable. Again, what if you get hit by a bus? Luckily, Wordpress itself can be transferred with just a database and web directory, including add-ons. Licenses may or may not be tied to a particular Wordpress account or email address, but a simple email to the theme's author should allow you to transfer ownership to the client (after it's been paid for).
If the client paid for a theme, they should get what they paid for. If you need to pay a fee every month, make sure the client knows that, and is taking over payments. Do that YESTERDAY.