Coinbase allows you to use a SandBox account where you can trade without real funds. This account requires creating API keys different from the production account.
To use the SandBox account, just set Coinbase.SandBox property to true, before doing any request to the REST API.
TsgcWSAPI_Coinbase oCoinbase = new TsgcWSAPI_Coinbase();
oCoinbase.Coinbase.ApiKey = "your api key";
oCoinbase.Coinbase.ApiSecret = "your api secret";
oCoinbase.Coinbase.SandBox = true;
MessageBox.Show(oCoinbase.REST_API.ListAccounts);