Voice123 is a marketplace that connects voice actors and clients. The casting process is free to use. The only cost involved is what you, as the client, pay the voice actor. When the casting process is over, and a voice actor has been selected or hired; any negotiations about payment, deadlines, and NDA's are between the voice actors and clients. Voice123 plays no part in the final transaction.
After the auditioning process, Voice123's system can be used as a communication tool between you and your chosen voice actor. It's up to the voice actor and you to negotiate the actual rate, possible buyout, and residuals.
Once you've selected the voice actor you want to work with, send them a message letting them know they're hired.