It seems to me that some sample DAE files from the model bank have complete transparent materials, so they are invisible. (Seymour.dae and files in the Skinning_ID_SID_Test)
Are they correct content?
If so, how can I render them to be visible?

                            <color>0 0 0 1</color>

Change either the <transparent> opaque attribute mode or the <transparency> scalar value.

Page 249 of the 1.5 spec provides explanations on opacity.