clones have to come from somewhere...seed at some point
Of course every plant eventually can be traced back to a seed, just as every human being can be traced back to two parents.
But just as every person (except for twins) is genetically unique, the same is true to some extent with unstabilized hybrid plants. All the offspring from two hybrid parents may be genetically different.
The "clone only" strain usually represents a rare select pheno from a variety of genetically different hybrid offspring.
In other words, you grow out 1000 Jack Herer plants, find one super-awesome pheno that distinguishes itself from most of the others by potency, scent, yield or some combination of the above, rename it "JoGro Gold" (or something else) and now you've got your "clone only line".
Most clone only lines are like this.
The problem now, is that crossing this plant to itself or any other hybrid plant creates another genetic roll of the dice. Whatever genetic traits made this plant special will get reshuffled randomly in the offspring, and the overwhelming majority of the offspring won't exhibit the exact combination that made the parent excel.
If you're damn lucky, you *MIGHT* find a ceed in there that is highly similar to the clone only parent. But such a pheno would still be rare, and not only is it impossible to identify this great ceed without growing it, you're probably going to have to grow dozens (or more) plants before you find it, and you may never find it.
Bottom line: With the specific exception of a few truebreeding lines that are called "clone only" (but probably aren't)
despite what the ceed sellers would have you believe, in the vast majority of cases, it is not possible or practical to stabilize clone-only lines into ceed form.
Assuming you know the difference, buying commercial ceeds that are supposedly copies of "clone only" strains is likely to result in disappointment. The ceed sellers get away with this, because most buyers, bluntly, don't know the difference.
Again, if you "must" have a clone-only strain, you have to get a cut of the clone.