I checked the licenses and they have sites assigned to them. Transferring them will un-assign those sites and might cause updates to become unavailable for your clients during the transfer. It's best that you notify your clients about this first.
Also, you are now allowed to transfer the licenses yourself which is recommended for your privacy and security. Simply go to your Licenses Page and click on the padlock icon beside your license to unlock it.
You can then create your client accounts and use each of the unlocked licenses for each account registration. Please also note that the account creation will also generate an email verification so in case you're not aware of this already, you should notify your clients to monitor their email for the Apex account verification.
If the account is already created or for existing accounts, you will need to login to the account you will transfer to and register the unlocked license.