WFRP, 2nd Edition, p.169:
Magic Items
Magic items are beyond rare in the Warhammer World. There are not
magical smiths churning out masses of magic swords in the forges of
Nuln. Each magic item is unique, with its own special history and
powers. A character with one magic item is considered lucky. The
mightiest heroes of the Old World might have three. (...)
(All emphasis mine.)
I don't have the other books handy, but I strongly suspect this approach has been rather consistent through the editions. We've always played it this way, at least: a simple, single magic item in WFRP is, imo, about the equivalent of a minor artifact in DnD in terms of value, rarity and importance.
1. None of the named magic items are found in the index...
... annoying, but it seems only the categories of magic item (rings, rods, staffs, wondrous items etc.) are in the index, not the specifics. Instead of using the core books to search for specific items, I recommend using one of the SRD websites - I use 5thsrd.org to quickly search for individual rules.
2. By default, you can't buy most magic items
Default 5e assumes that "magic shops" pretty much won't exist, and even goes on to suggest that potions and the like be bought at alchemist's shops instead.
From the DMG, page 135:
Unless you decide your campaign works otherwise, most magic items are
so rare that they aren't available for purchase.
and further:
If your campaign allows for trade in magic items, rarity can also help
you set prices for them. As the DM, you determine the value of an
individual magic item based on its rarity. Suggested values are
provided in the Magic Item Rarity table.
The Bag of Holding is an uncommon item, therefore in the 101 - 500GP range. At this point, it is at your discretion.
Magic items in 5e
Unlike previous editions (particularly the 3/3.5/4 era) 5e is designed to depend much less on magic items. Together, the:
- attuning rules (which limit the number of powerful items any individual character can have),
- the expressly "lower magic" setting (which generally restricts the purchase - and sale! - of magic items),
- and their "bounded accuracy" (lower difficulty, ACs and progression gradients)
...ensure that high level 5e characters are powerful because of their raw skill and ability, not the plethora of magic items they happen to be decked out in. This was a specific design decision going back to the very first D&D Next playtests. (As a side note, having your party find that Bag of Holding in some dusty and forgotten attic is also more rewarding than just stopping by the local Bags of Holding R Us and grabbing a few!)
Best Answer
This is explained in the Rarity section of Magic Items on page 135 of the DMG:
Aside from being a rough measure of power, rarity, as somewhat stated above and by its very name, also provides a guideline for frequently characters might find a certain type of magic item.
A legendary magic item is far more powerful and likely to be far more scarce than an common or uncommon magic item. Characters might find multiple Cloaks of Protection (uncommon), for example, throughout their adventures but would probably only ever find one Robe of the Archmagi.
Depending on the type of game your DM runs, obtaining magic items may be a rare occurrence but that still doesn't necessarily mean that all magic items are rare or that there's only ever only one of a magic item in existence.