So how did Smaug carry all of the Gold and jewels to the mountain lair from the kingdoms and towns that he had annihilated? He doesn't seem to have any buckets lying around and I don't reckon he could pick up much with his big claws?
It's canon (not only in Tolkien but in traditional dragon lore generally) that dragons do this, but nobody says exactly how. Maybe they just swallow any treasure they find and regurgitate it when they get home? Many species of birds do this to feed their chicks in the nest, and as birds are very closely related to reptiles it's not too far-fetched an idea.
It's possible, too, that the acids in a dragon's stomach are so strong that gold, silver and hard crystal gemstones are the only things they can't break down. That would mean that a dragon didn't need to sort laboriously through the loot; he could just swallow the lot and when he got home only the really good stuff would be intact. (He'd lose any pearls, of course, but when you have rubies and diamonds, who needs pearls?)
Did you ever read Peter Dickinson's book The Flight of Dragons? He suggested that dragons were essentially a kind of bio-airship, that most of their bodies were a hollow space filled with hydrogen created by hydrochloric acid in their digestive tract (and that the fire-breathing was essentially just draconian bad breath, excess acid being burped up and burnt off).
Dickinson also suggested that the habit of gold-nesting had the same cause: that dragons continually leaked such corrosive vapours they destroyed anything corruptible they touched; so gold was almost the only material they could go to sleep on that would still be there when they woke up!
I've never encountered anything else of his, but I did like this. Sadly, someone who knows more about chemistry than I do (not difficult!) says Dickinson's suggestion for the process by which dragons produced hydrogen isn't feasible. A pity, because the theory actually solves so many of the problems and mysteries created by traditional accounts of dragons, e.g.:
Traditional dragons are immensely too large for their wings - or any wings imaginable - to keep them airborne. - Their wings didn't. It was the dragon's lighter-than-air body that kept it airborne (and a lighter-than-air vessel needs to be huge); the wings were just for steering.
If dragons existed, why are there no dragon fossils? - Because dragons created a hydrogen supply by continually and rapidly growing bone spurs in their gas chambers and metabolising hydrochloric acid which dissolved the bone into hydrogen. A layer of mucus isolated this violent chemical activity from the rest of the dragon's body, but when a dragon died the mucus broke down and the acid simply destroyed the skeleton.
Why is dragon's blood universally agreed to be poisonous, and so corrosive that it frequently destroyed the weapon that killed it, and killed the dragon-slayer if he got deluged with it? How can any animal have poisonous corrosive blood? - It wasn't the animal's perfectly ordinary reptile blood that did all that, but the acid.
How come such a huge and powerful creature could so routinely be killed outright by a single blow into its breast or side with a sword / arrow / lance? How come we never read of such a blow just wounding a dragon? - Obviously, once the dragon's gas chambers were punctured, the acid would spurt out and start corroding its body. It might limp home to its lair and take a while to die, but it was a done dragon.
Dickinson also suggests that only male dragons flew, breathed fire, and laired in caves, and that female dragons were flightless and aquatic. This, obviously, accounts very neatly for Grendel's mother, the Lambton Worm and the whole gallimaufry of other reptilian monsters that traditionally lived in meres, rivers and wells.