I use light armour, so I don't really have any reason to cart around dragon bones (since I'll be crafting armour out of dragon scales instead), except that they're ridiculously valuable, and I need the gold.

So, who can I sell them to?

General Goods vendor

I must be going to the wrong ones, then.

Uh, not sure, have you tried selling em to some blacksmith? They're under smithing, so logically blacksmiths would buy them. Those general goods store probably would as well, or have you tried all of them out already

The general goods store in solitude bought some last time I checked.
The traveling Khajiit traders probably do too.

The skyforge guy might...he's the only master level blacksmith in skyrim....

Only General Goods vendors will buy them. This includes stores such as 'Bits and Pieces' in Solitude, and 'Belethor's General Goods' in Whiterun. Smiths will not buy them.

The Blacksmiths don't.
Only misc ones like ingots/ leather.

So far I've just seen General store ones buy those.
One in Riverwood when you fast travel on the right
before the Inn