What Compendium entries cannot be bought from Purah's Assistant?

Solution 1:

After defeating Ganon, and entering the post-game, the following extra pictures can be purchased at 500 Rupees per picture, in the "Elite Enemies" section:

• All Silver Enemies (e.g. Silver Lynel, Silver Moblin, Silver Bokoblin, etc.)
• All one-time only enemies (e.g. Bosses)

The following picture can also be purchased for 100 Rupees, in the "Weapons" category:

• Light Bow

Solution 2:

After reading the comments on my question, I realized I had purchased all the pictures after facing the final Divine Beast and before fighting Ganon. Turns out I had two pictures that were available to purchase; a weapon and an "Elite Enemy" picture, a new category with a gold icon that was not there before I fought Ganon. The weapon was the Light Bow, and the Elite Enemy was Master Kohga. Upon purchasing these, Symin informed me that I had obtained every picture in the compendium. So it turns out there is an opportunity to purchase previously unbuyable pictures post-game.