Gold credits can only be purchased for real money by spending users and can be converted back to real money by Sine Wave virtual goods partners.
Gold credits can be purchased at a approximate 100:1 ratio against the USD ($). They can be redeemed back by registered creators for a similar value, less transaction, processing and marketplace fees (see the creator agreement).
Silver credits are free promotional credits given to users as rewards for participating in the community.
Silver credits cannot be converted to real money but can be used by creators to promote their content to new platform users who have not yet purchased gold.
You can set each item you want to sell to gold only, silver only or any combination of the two.
As a guiding principal we would recommend making most of your inventory gold only but offering a few items for silver as well, perhaps 10% of total inventory, to help promote your goods to a wider audience and to give the platform a richer pool of content for new users to explore before they buy gold. Silver credits can be used to offer 'freebies', without devaluing your brand or your items.