The best solution would be the addition of a "junk bag,"
You can already do that. Place items you want to sell in a basket/crate, and then sell the container all at once for the value of the contents.
I see what you're saying, but each container only has a 20-item limit (unless that's been removed in the most recent patch?), and you'd also have to open a container and manually drag each item into it every time you want to mark it as "junk."
What I'm talking about with a "junk bag" is something much more elegant (IMHO), as seen for example in the most recent Spiderweb games (e.g. Avernum:EFTP), where you can "CTRL-click" an item and send it automatically to a functionally infinite loot bag for later one-click sale. Of course, D:OS has an inventory weight limit for non-equipped items (unlike A:EFTP), so it probably wouldn't work with this game as Larian intended it, but I personally love that feature as another way to reduce time spent with tedious inventory management and allow more time to simply enjoy the game.
I use backpacks to store important +skill gear and scrolls, baskets to store crafting reagents, and barrels to store vendor junk. That way I never accidentally sell a container full of important things to a vendor.
This I do, though - it's at least a good Plan B until/unless they implement proper inventory sorting (and prevent random inventory re-sorting in trading screens).