My ally and I are in combat, he is unconscious and (per an agreement made before passing out) wants me to pick up his most valuable things (a pearl, some gold, ink, his spellbook and a spell scroll). I know that he has them, but I do not their exact location (except that they are in their backpack).
My question is: do I have to succeed at a roll or do I just consume my action and take all the items?
This is largely up to the DM and depends on a lot of variables.
Picking up items that the character has out and are obvious (if he have the spell book in hand and was reading from it) should be a free interaction, as it is now just lying on the ground next to them. Some DMs might say they fell on top of it and it would take an action to search under their body.
Other items, like a pearl that is tucked away, would require rifling through their bags and pockets to find (unless for some odd reason you had the exact knowledge of where they keep every item). If you think of turns as six second windows, and imagine looking for a pearl in a bag full of stuff without knowing whether or not it is even in there, you could see how this might take several turns.
As a DM, I call this "quick looting" and have the player roll an investigation check to rifle through. The higher the roll, the more they get, but it takes their entire turn. The only way it wouldn't is if they had specific knowledge of exactly where something was, or if it was really obvious (like a large potion that took up 1/3 of the bag), in which case they would just get the item but still take their turn doing so.
This has usually been used for taking potions of healing from allies and enemies that they have seen using them. It has, in my experience, provided an adequate amount of cost for the reward. Skip your turn but get an item that you want for next turn (or bonus action chug that potion).
PHB 190 states that picking up more than one object will require an action:
You can also interact with one object or feature of the environment for free, during either your move or your action...If you want to interact with a second object, you need to use your action.
Since you're taking multiple objects, it should require an action.
However, DMG 237 says that you should only call for a roll when there's a chance of failure:
When a player wants to do something, it's often appropriate to let the attempt succeed without a roll or a reference to the character's ability scores. For example, a character doesn't normally need to make a Dexterity check to walk across an empty room or a Charisma check to order a mug of ale. Only call for a roll if there is a meaningful consequence for failure.
Since you've already made arrangements with your ally to take his stuff, it seems pretty clear that taking those items is easy enough that there shouldn't be a risk for failure.
Further evidence for this conclusion is the "interacting with objects around you" list on PHB 190. This list gives some examples of what you can do with your free object interaction, and includes the following entry:
withdraw a potion from your backpack
I don't think that taking an object from an unconscious friend's backpack is substantially different from that example.
