ha ha, yep.
OH! If you're starting to collect the knuffel, then just in case you haven't figured it out yet I'll make sure you're alerted -there are two different kinds of elemental knuffel in Knufftropolis.
The "trial" knuffel will be permanently available, and never run out.
but the other elemental knuffel, the ones that say "generation ##" on them? Those are around for a time and then become retired.
After that, the only way to get them is by clicking the "older generations" button in Knufftropolis and buying them for

or to get them from other users.
(sometimes another user will 'abandon' an older generation and it will show back up in Knufftropolis but that's pretty rare, and ultimately it's still from another user)
You *can* trade knuffel now, but that's new, and you can't list them in the Market so you'll only see them go via forum trading.
So, yes.
If you're ever hesitating between an elemental knuffel of the trail type or the generation, know that you can typically trade a retired generation knuffel for a couple of trial ones, if you have the patience. So that's a factor.
It used to be the generation ones had a limited number available, but I think now they're just however many get taken before a set date they retire on. (Or, month, not specific date in that month, but still).
So for instance the current Darkness Gen 21 knuffel retires in December of this year -next month.
You'll probably want to snag one of those before they go.