Sometimes the second-hand market shifts in an overt manner, like how the Vanquish Soul cards got a boost due to their synergy with the K9 cards from Justice Hunters. Sometimes, however, price spikes can occur quietly in the background. This is exactly what has happened to Shaddoll cards over the past month.
Shaddoll Squamata

Shaddoll Squamata started last month at just under a dollar. This isn’t surprising for archetypes largely forgotten or relegated to the lower end of rogue strategies. At the end of July, copies of Shaddol Squamata began breaching the $1 average mark.
Now, a week into August and copies are exchanging hands for around $5 on average, with the occasional copy sometimes jumping to around $8. Based on the current TCGplayer analytics, it looks like the trend is still heading upwards.
Shaddoll Squamata is one of the archetype's main targets for its fusion spell. While its targeted destruction is relatively fair for a flip effect, the ability to drag another in-archetype card into rotation is a lot of value.
Shaddoll Beast

Copies of Shaddoll Beast largely followed the same story as Shaddoll Squamata. Using the same timeline, Shaddoll Beast started July just over a dollar before shooting up, in the last week, and it’s now selling for around $6 - $8 on average. Like Squamata, there's a steady upward trend for the time being.
As another of the main deck staples for the Shaddoll archetype, Shaddoll Beast is simple in its execution. While a player can easily send a copy to the graveyard for a simple draw, getting to flip Shaddoll Beast face up easily gets two cards into the hand while cycling a card into the graveyard.
The fact that the cards are drawn instead of added safely bypasses the usual hand traps like Ash Blossom & Joyous Spring.
Shaddoll Fusion

As the main vehicle of accessing the Shaddoll fusion monsters (while moonlighting as a double Foolish Burial if the opponent has an extra deck monster on the field), we expected copies of this card to at least match the movement of the other Shaddoll cards in the second-hand market. To an extent, it did.
Starting off just under a dollar, the market average for copies of Shaddoll Fusion is now around $10. As if this high price point wasn't enough, recent sales have topped out at $11.
Nehshaddoll Genius

Copies of Nehshaddoll Genius had the same humble beginnings as the other Shaddoll cards, starting at just under a dollar in value. Where it differed was that, into mid-July, copies skyrocketed to $8 on average, where it’s been holding steady ever since.
Even amongst all the Shaddoll cards covered in this article, this particular one has moved the most copies in the past month, over five hundred copies worth.
While we don’t have any solid evidence for this unique market shift, we could probably attribute this to the single printing of Nehshaddoll Genius back in 2019 from the Ghosts of the Past expansion. All of the other Shaddoll cards on this list have been reprinted at one point or another.
As for Nehshaddoll Genius's contributions to the archetype as a whole, most players may feel that this card is more of a tech choice as its flip effect is marginally leveraged. That said, it looks like some players may be looking at Nehshaddoll Genius’ secondary ability of temporarily turning off any effect monster on the field for a turn.
Despite being temporary, Nehshaddoll Genius’s effect is quite a board breaker going second. You don’t even need to have it in hand because of Shaddoll Fusion or even be the main target of the fusion if you happen to use Shaddoll Squamata.