The default payment gateway built into ClassiPress is PayPal, with an option for manual payments also available. You may be interested to know that there are changes being made to the payments functionality in the next major release,
ClassiPress v3.3 which will allow the theme to be compatible with other payment gateway plugins available in the
AppThemes marketplace. At this time these plugins are not compatible with the existing version of ClassiPress, but the release will change that. Outside of this, you can implement your own payment gateway and there is an
available tutorial here which explains the process. Thanks.