I guess the search to find a price on amazon.com is not specific enough.
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For example "Conclave Equenaut" is currently listed as $24.99. This is a simple common Ravnica city of guilds card, should cost not more than a few cents.
If you manually search "Conclave Equenaut" on the amazon-site in "Toys & games", you will find 4 entries: 1 for the regular card, 1 for the foil, and 1 for the card from a duel deck - all listed for a few cents.
However, the 4th item dat pops-up when searching for conclave equenaut is "Set of 137 Varied Ravnica City of Guilds Cards (Magic the Gathering: Ravnica City of Guilds Common, Uncommon)" for $24.99. The description of this item lists dozens of specific cards, including the conclave equenaut.
For the same reason quite a few other cards from Ravnica City of Guilds will be (wrongly) listed as $24.99.
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Another example:
Unlikely Alliances, listed as $95.99, should not be more than a few cents.
Searching on amazon.com in toys & games will return not only the mtg-card but also:
"Dragonball Z Unlikely Alliances Action Figure 2-Pack Vegeta & Frieza $95.99"