DataGridView.SelectedCells is a collection of cells, so it's not as simple as calling ToString() on it. You have to loop through each cell in the collection and get each cell's value instead.
The following will create a comma-delimited list of all selected cells' values.
TextBox1.Text = "";
bool FirstValue = true;
foreach(DataGridViewCell cell in DataGridView1.SelectedCells)
TextBox1.Text += ", ";
TextBox1.Text += cell.Value.ToString();
FirstValue = false;