Are non-useable Oddities anything I should be afraid to sell or are they just really SHIN disquesed as purposeless junk? Like should I sell every Oddity I find?

I tried to keep most of them around as long as possible, just to see how much unique junk I could find. There are one or two that have more use than just junk - however, these should be obvious (sort of) since you can actually USE them.

Well, when I say "more use than just junk", in most cases it's just junk with more detailed descriptions; however there's one that is useful for a puzzle - which, upon solving, rewards you with... a cypher of some type. So you're not really missing anything either way, considering how common cyphers are.