How can I let users purchase phone numbers without having credit on my account? I'd like the users to pay for the numbers directly themselves. Is it possible to have the users payment sent to my account as a credit and then use it to pay for the number? How can I do this with Twilio API?
Phone numbers are purchased from your account. So you need to have atleast 1$(i think 1$ is required to purchase a phone number) of twilio credit to purchase a number. If you didnt have credit, you cannot purchase a numnber. And i think no Api is available to credit your account.
Best way to implement twilio number purchase webportal are 
1) Have some credit in your twilio account
2) Charge users when they purchase twilio number
3) Set a twilio recharge trigger so that your twilio account is recharged from your bank account when credit goes below a limit
How can I let users purchase Twilio numbers from my site?