Items are pretty important in this game if you don't want to die right away. You need potions and jewellery to survive this game.
Health potions are what you buy and sell the most. If the enemy takes a long time to beat, be ready to use a lot of health potions until the enemy is defeated.
Magic potions are the same, but for Adrianna. Since she's the sorceress, you don't want her magic to take a while to regenerate, especially when enemies are attacking her due to her weak strength.
Recall potions are important in terms of transporting in between the mission and the shopkeeper. It's a good idea when you reach a maximum weight to sell some stuff and buy more powerful items to help you in the game. Rest assured, you won't lose your progress when you recall back to where you left off. If you are Adrianna, you really need lots of recall potions.
Rings and necklaces are important for your stats. For example, if Adrianna has weak strength in Act 3, she can use the ring with +5 strength to boost her combat and her weight. But if the ring or necklace has no stats, then it's useless and needs to be sold. It's a good idea if one of your stats has been neglected and you need something to help boost it.
Other items like gems are useless in this game. The only thing you can do is sell them to make more gold. So if you need money for an item, sell the gems to lose the carrying weight and hopefully you will have enough for it.