One would also assume that they could...
Craft some items themselves.
Gifts from GREATER beings, when the dragon defeats a great evil.
Take humanoid form and adventure.
Take humanoid form and wed, gainning the inheritance.
Be entrusted with evil artifacts, from "evil" magical toothpicks, (damn superstitious farmers), to a talisman of ultimate evil.
With age and curiosity, a dragon could have quite a library of spells, even if unable to cast them all. A complete work of every Wizard, Cleric, Druidic, and Bardic spell would equal dragon size bragging rights.
Trade for items that other spellcasters create for a chance to browse said spell collection.
Coin is a humanoid invention. Simply finding a cave lined with veins of silver and gold, them altering the cave to bring them out in all ther natural glory would be quite beutiful. It would probably mean more to a good dragon than a lump of gold bits with dead human kings on them.
Friends. Any creature as longed lived as a dragon, yet cautious about relations with its own kind, would value friends above all. A dragon finds comfort on the road in the home of a decent average halfling, and over the years they become friends. He then becomes friends with said halflings children, and so on and so on. After a couple of generations, he may well have seen the birth of every living person in the village. It would not be hard to imagine Gandalf as such a wandering dragon.
Odd circumstances. I just had a thought about a really bored dragon sitting on a pile of gold bits depicting dead kings. He looks over and notices the flimsy little rod of wonder he somhow got talked into taking as a trade. Just to see why some deranged little wizard would craft such a thing, he picks it up and activates it.
Then he does it again.
And again...
And again...
After a few solid days of boredom, who knows what that thing could spew out.