A lot of items as part of a multipack even say "not for individual resale" on them. Some product manufacturers even go out of their way to not put individual UPCs or nutritional values on the items.
You won't go to jail if caught, but can be cut off by your supplier, or even a retail store you're buying from.
Which is pretty much the same thing WoW is doing.
Like it or not, WoW is a commercial product, and the owning company has every right to profit off of it, even through shop sales, and the customers do not.
That's business, and it doesn't matter if someone agrees or not.
And there's more items you can own but not be allowed to sell than you might think. Medications and medical supplies being a big one. Computer programs and things with licenses and contracts. Much like the one you sign with WoW when you agree to ToS.