Storage and junk items?

[Estellios]

As Urthdigger said, if you don't need it in the immediate future, get rid of it. All the stuff you are getting now is easy to get if you actively hunt for it. Everything that is rare or difficult to acquire will be likely be priced appropriately.

For the record, very few fish are used in crafting (it will say Crafting Material in the description) or in quests so it's not worth holding onto them, sell them.

As far as appropriately pricing items you yourself sell, it's best to take a look at the price history and see what they have been going for. It's usually more profitable to sell items at prices people are willing to pay than to try to raise the price and squeeze more cash out of the buyer, except for extremely high demand items.