Is it legal/ right if I create a texture pack of about 20 downloaded textures and 20 normal maps, and then sell it on an online store? All the textures are CC0 licensed.
CC0 is a mechanism for an author to give up as many rights in a work as possible, in order to cause a work, virtually or actually (if possible), to enter the public domain. It either releases a work into the public domain by waiving copyright, if that is a valid action in a given jurisdiction, or else licenses the work with extremely broad permissions.
Assuming that all of the works have been validly licensed (or surrendered into the public domain, as applicable) by their copyright holders, then you may treat the work as if it were in the public domain. That means that you may include it in a collective or derivative work, and redistribute it, at cost or for free. Note, however, that someone who buys the CC0-licensed textures is also free to redistribute them further, since the same lack of restrictions applies to them as it does to you.
If you modified the CC0-licensed textures before distributing them, then you hold a copyright in your modifications in the resulting derivative work (assuming your modifications are sufficiently creative and substantial to qualify for copyright), so you could enforce copyright in that case.