If a sack has gameplay value, then fine. But OP only talked about using it as a mechanism for storage and trade. My contention is that as a mechanism for trade, there are better systems, and as a mechanism for storage it sounds pretty boring.
Having to make tradeoffs about what you can and can't carry is a worthwhile game mechanic, but it would be silly to say that you can't carry certain 'loose' items in you big sack, simply because you lack a small sack to place them in. That's just complication for the sake of complication. The only type of character I can see benefitting from arbitrary sacks would be some kind of merchant/trader, but even then the sack itself, as an item, isn't terribly valuable. Just give the ability to create sacks to the classes or designations that need it.
When I play Diablo3 I find myself grouping gems and other like items together. Perhaps that's just because I like to play around with my inventory.
Therefore, I think that an inventory system should support container items. I could for example have a large sack that contains a few potions, a book, and a small sack full of gold and gems. Having container objects would allow the player to store items at his character's home. The player wouldn't need to litter the floor with hundreds of items or use a barrel to dump all their potion ingredients in.
Personally, I like items in rpg games and I tend to hoard them. Even in my Skyrim game I have a house with one of each weapon and armor in it. One thing I wanted in Skyrim was a pack mule. If I had that, I would gladly spend the time emptying the dungeon of every trinket (including the dead bodies) I could find.
I really think that success or failure of such a system would largely depend on how much the game automates these tasks for you. If the game can automatically organize our inventory I don't think it would bother those casual gamers who hate inventory management. Perhaps automated inventory management system could be a game option.