type

The type object specifies which trade mechanism is used. For example the item can be sold for cryptocurrency, or it could be available for swapping with another item.

FREE

The item or service being offered is free. The cryptocurrency field is not required if this type is set. If it is present, then it should be ignored.

"payment": {
    "type": "FREE",
    ...
}

SALE

The listing is just a typical sale, the seller offers an item or service in return for a specific amount of cryptocurrency. The cryptocurrency field is required.

"payment": {
    "type": "SALE",
    "cryptocurrency": [
        ...
    ]
    ...
}