Where is the line between sellable and unsellable?

[Kinne]

Seriously, I get good weapons for selling, and they're unsellable. For no reason at all. No explanation. What makes unsellable weapons unsellable? They aren't "key" items like the oni dagger or onigadachi, I got them from enemy drops. So why can I only sell the crappy ones with rank 1 skills that nobody will buy?