Can I safely sell all items marked as treasure in Recettear?

I'm fairly certain that you are safe selling anything and everything you find. Most items are obtainable multiple times, and even the unique ones dropped by bosses will drop again if you go through that dungeon again.

In the case of the Very Odd Vase, that is only dropped from the boss on the 15th floor of Jade Way, and randomly purchasable through customers, making it a bit uncommon. While I'm pretty sure that the vase isn't used for anything important later on in the game, you may want to double check the Recettear Wiki's Fusion Table before selling items, as it could be a pain in the neck trying to find these useful items again after you've sold them away.

If you're ever unsure about an item's value, and you aren't still in the middle of the debt-paying phase, you can always keep the item on hand. Apparently each saved game has a maximum limit of 15,000 items, so hoard away!

Like you, I have yet to finish the game, so others can feel free to correct me if I'm wrong in assuming that all items are re-obtainable.