If you want to move emails from INBOX to trash, select INBOX (source) folder first.
You may also use CommonFolders class to identify IMAP's server trash folder.
using (Imap imap = new Imap())
{
imap.Connect("imap.example.com"); // or ConnectSSL for SSL
imap.UseBestLogin("user", "password");
// identify trash folder:
CommonFolders common = new CommonFolders(imap.GetFolders());
// select source folder:
imap.SelectInbox();
List<long> uids = imap.Search(Flag.Seen);
// move to destination (note the bulk method usage):
imap.MoveByUID(uids , common.Trash);
imap.Close();
}