This implementation requires you to set up a line of credit with Facebook. Please contact your Facebook rep to initiate this process.
Under this model, the ad accounts are owned by you but mapped to customers (on your end).
While you don't have to build interfaces to have the user set up the ad account, the page that you advertise off should still be owned by the advertiser.
Because you own the ad accounts, you are responsible for the ad spend that accrues on the accounts. You should consider having your platform charge the customer ahead of time on ad spend, so you are not at risk for spend.