There are none currently in the shops. Are they rare for some reason (eg, should I not be feeding them to the Recycling Beast?) Or is this just a fluke?

Either everyone's fed all that are on the market to the recycle beast (kind of unlikely) or the item's a bit glitchy much like some other poor items you can't search for...

:(

As far as I'm aware there's no new achievement or anything making them rarer/more valuable :)

That item along with the slot items and like the items showed up there are all glitched. If you hit search a few times you may find one or two that some how made it through, but normally none show.

Ah, fair enough. (Odd that only happens to some items).