You can sell Plugin by yourself
Your zip file must have plugin and or paymentgateway and or theme and or device folder
Example Repository
Example structure for plugin.zip
├── plugin/
│ ├── plugin.php
│ └── ui/
│ └── plugin.tpl
├── changelog.txt
├── install.txt
└── license.txt
Example structure for paymentgateway.zip
├── paymentgateway/
│ ├── payment.php
│ └── ui/
│ └── payment.tpl
├── changelog.txt
├── install.txt
└── license.txt
Example structure for theme.zip
├── theme/
│ └── theme_name/
│ ├── home.tpl
│ └── order.tpl
├── changelog.txt
├── install.txt
└── license.txt
Example structure for device.zip
├── device/
│ └── deviceName.php
├── changelog.txt
├── install.txt
└── license.txt
or you can make full package
Example structure for fullpackage.zip
├── theme/
│ └── theme_name/
│ ├── home.tpl
│ └── order.tpl
├── paymentgateway/
│ ├── payment.php
│ └── ui/
│ └── payment.tpl
├── plugin/
│ ├── plugin.php
│ └── ui/
│ └── plugin.tpl
├── device/
│ └── deviceName.php
├── changelog.txt
├── install.txt
└── license.txt
install.txt instruction to install manually
license.txt is your license for the plugin
all the files inside folder plugin and or paymentgateway and or device will be move to PhpNuxBill system, and theme will be move to ui/themes