The best plan is to create a new Particle account to use for development of the project you’re transferring. Then, when complete, you can just change the account email to the new owner. When you do it that way, all of the devices transfer and stay activated, the product ownership is transferred. Any code in the WebIDE will also transfer, along with webhooks.
If you have SIM cards the credit card would normally go with the account but you can submit a support request to delete the credit card so if the new owner does not update the credit card before the next billing cycle, the devices will stop working.
It is possible to move some things one-by-one, but changing the account email is usually the easiest method.