Hi Andrew,
I would use the following based on your code:
Dictionary<string, object> parms = new Dictionary<string, object>();
arms.Add("&&events","event1" ); //set custom events
parms.Add("&&products", string.Format("{0};{1};{2};{3}", productType, productId, quantity, dollarValue)); //set product string
ADBMobile.TrackAction("purchase", parms);
Thanks,
Asheesh