I think you've hit the high points, but...
Collectibles, such as antiques. Generally, these have low value except to a few buyers that recognize and value them, and they tend to be fragile. Objects of art which are not made of expensive materials tend to be similar. Religious artifacts tend to be similar, in that you'll get a lot more for them from someone that considers them sacred artifacts or relics, than you will from anyone else. It's just a bunch of bones to most people, but properly preserved with the graveclothes intact, to the cult of Foo, this is the bones of St. Barweetot. Likewise, consider mementos like a simple silver portrait locket. It might be worth a couple of g.p. to an average person, but if the Duke recognizes this as a locket that belonged to his beloved sister that contains paintings of his mother and father, then he's likely to pay hundreds of times more than an average merchant would, because to him its nearly priceless.
Trade Commodities. These are things like bags of coffee beans, hides of copper, ingots of glass, bolts of linen cloth, jars of olive oil, barrels of wine, and the like. Collectively, they can represent a lot of value, but the value per pound tends to be fairly low. Weapons and armor tend to fall into this category. For example, 50 longswords or 50 suits of mail.
Live Animals/Monsters. This is an AD&D classic. Quite often the most valuable treasure in an encounter is the monster itself, or its young or eggs.
Dead Animals/Monsters. If you can't take them alive, then they are often most valuable cut up in pieces. However, you have to be skilled at butchering if you want say an intact hide, and will need some way to preserve the specimen if you want to sell a dragon's heart to an alchemist making saying potions of super-heroism.
Frescos, mosaics, and other architectural features. Yet another AD&D classic - the dungeon itself is the treasure. Now you just have to figure out how to deconstruct it and transport it.
Note that the following are different categories:
a) Hard to identify - Things that don't look valuable, but which are.
b) Hard to get - things which are hard to find, remove safely or intact, hard to reach, etc.
c) Hard to transport - things which are bulky, heavy, and/or fragile.
Of course the three categories could overlap. "Buried in the mud is a wine bottle with a faded label. It's actually a 100 year old bottle of a legendary vintage, and surely one of the last in existence."