Do miscellaneous items take up inventory space

Merchants have a basic inventory that never changes that consists of common and miscellaneous items, but their uncommon and up inventory changes.

I don't know the exact details, because it seems fairly random due to sometimes them changing when I leave for 1 second, and sometimes not changing for 10 minutes.

I'd hazard a guess at one of the following, and I'll clear this up once I've researched it more:

1. It's similar to dead bodies despawning in that if you can't see them for a limited amount of time, then they despawn, maybe you just have to leave the immediate area for x amount of time.

2. I think this is the more likely one, perhaps there's a hidden timer that causes them to change their inventory similar to the special deals in Borderlands. Say every 15 minutes the inventories change, whether that's 1 minute from when you see them, or 10 minutes from when you see them.

Some will be used for mods, most you can sell. I played this for PS3 and had TONS of blue and violet weapons. I sold almost every green weapon or item I came across being I had either found or made enough weapons that were way more powerful than any of the green stuff I was getting.

Also, side note, I had at least 3-4 orange, only liked one of them.