There are 2 types of sandbox servers: The Old Sandbox,
The original Sandbox was a website that housed a version of MooMoo.io that allowed for infinite items to be placed, and you can go into negative resources. In its current state, the Sandbox is very outdated and has not been updated since its initial release. As previously stated this version allowed for an infinite number of all items that were currently in the game to be placed. An exception is the Mine, which was removed from the game for unknown reasons. Also, There is no windmill on Age 1. They were probably removed to prevent trolling, and easy aging. It only got half of the 0.32 Update and stopped updating after that. Also, you can't go into negative Gold. The Old Sandbox has since been removed.
Old Sandbox Link: <http://moomootest-env.us-west-2.elasticbeanstalk.com/>