Final Fantasy 15 – Are There Moments When Some Sidequests Become Unavailable or Fail Automatically?

There are, in fact, certain treasures that are used in quests or to upgrade weapons (also considered quests).

I found a list of all of these "Don't sell" treasures in the game here:

• Barbed Scythe
• Cactuar Needle
• Coeurl Whiskers
• Cylinder
• Dynamo
• Earth Gemstone
• Glass Gemstone
• Magitek Core
• Magnetron
• Metal Scrap
• Monster Claw
• Repair Kit
• Rusted Bits
• Sky Gemstone
• Spiked Armor
• Sturdy Helixhorn

Thus, it looks like all other treasures are able to be sold without worry.

From my experience, you will be unable to parry (despite the prompt making you think that you can) if you are still carrying out an attack. This has been the most common reason I've failed parries.

1. I can confirm that what direction you are facing will not prevent you from being able to parry an attack, as I've parried attacks that were coming from behind, simply by holding the Square button. As long as you have the Square button pressed when the attack hits you, you should parry successfully.

2. The time window varies with enemies, as they attack at different speeds. For instance, I've parried the Slime-type enemies (Flan, etc) by holding down the Square button as they approached and it's taken up to 10 full seconds before I actually parried (if not more) as they approach very slowly. For faster enemies like a Coeurl, you will have less than a second to react, and, from experience, if you're mid-attack at all, you won't make it. Using quick weapons like the shortswords will give you a better chance to successfully pull off a parry. Using a greatsword, on the other hand, results in very few successful parries.

3. As to whether or not parrying is worth it, definitely. I have survived many fights that my party members have all died during simply because I stopped attacking and just parried everything I could.