Is it better to donate or sale the cheaper more common items that I get from daily quests? What sort of items should be donated? Would selling the common and mundane item at or below MSRP be useful for other people that are trying to complete quests?
I believe most people are selling their quest items or they are fragging them over at Darkside for easily earned Crystals. There is no standard best way though, everyone has to find their own preference there.
As for donating, there are no rules for this and even junk items are snagged from the DC at some point. Free stuff is free stuff after all.
As far as those go, everyone does it differently. There are no set "rules." Quests give you a generally random set of items, though some have standard items (like Random) that are so common, their values are already extremely diminished.
In short, you're not going to get in trouble by any stretch of the imagination for getting an item you don't necessarily want in a quest, turning around, and selling it or donating it.

Thank you, I hadn't though about using them as fodder for the Fragmentizer.
Thank you, for the insight.
There is also the Recycle Beast who often asks for those items mentioned. So keeping them stored to use when they show up, or donating them, so those of us who do hoard can snag them, is all good :).
In loving memory of Need posting achievements?Then join Posting Frenzy Achievement Items - searchable list List of Borders and cutouts New at Fresh and Flavorful Ping Group
I'd advise against selling very cheap/common items, purely because it seems like a waste of time and effort to hand-price that stuff for the low financial return. But only you can decide if it's worth it to you!
Personally, I'm a huge hoarder, and as far as I'm concerned there's no such thing as a "junk" item on Subeta :D no matter what you choose to donate, someone will appreciate it! I have dozens of pages of nearly-worthless stuff in my vault, just waiting for the day the Recycle Beast asks for it. Though last week I finally decided it was long past time I do something productive with at least some of it, and fragged tens of thousands of items (result: one shiny new background, many back-issues of the Subeta Tribune, and over 20k crystals still in the bank). Of course the Recycle Beast decided to mock me, and asked the very next day for a cheap wearable I'd just fragged a couple of hundred of after they'd been sitting in my vault for several years... :S
Since you phrased it as a point of etiquette, out of curiosity I tried to imagine something you could do with cheap items that would actually class as annoying or obnoxious... Then I remembered reading someone's suggestion on what to do when Major Drills wants you to send a gift to someone: just find a random person and anonymously send them any old bit of cheap tat you want rid of. Now that, I thought, was pretty tacky; YMMV I guess (clearly some people are okay with it!), but if you want to be on the safe side, maybe don't do that. Anything else you can think of to do with your stuff? - absolutely fine and dandy. :)
I'm in agreement with Phlizz. It's not really worth reselling any extremely cheap or common items.
But if you are looking for some quick sP, I also suggest heading over to the Pawn Shop and seeing how much you can get out of the items. Usually I would pawn those items that can get me at least 1,000 sP and upwards. (Some of the __HP healing potions can be quite worth it, for example.) It's ultimately not a lot, but hey, it's still quick and easy sP, and you can still earn something rather than losing any sP potential by fragging everything. It will add up :D. Anything else left over can be donated or turned over to frag.
Good luck!
[img align=center]https://media.tenor.com/oLk4aQyJggoAAAAC/ace-ace-kpop.gif[/img]
Want a bite?
I bite for: 56 strains
Bite Me/In progress: