What about the CheckedItems property found within the CheckedListBox component? From your code, I'm not sure how you go about adding each Account object to a CheckedListBox so to answer the second part of your question, here is a simple example of how to use the CheckedItems property for a generic CheckedListBox:
foreach (var item in myCheckedListBox.CheckedItems)
{
//Perform some logic for each item which is checked.
}