what textures can/can not be used?

Correct, except you can use textures.com as long as you mention you sourced your textures from them not selling them as apart of a texture pack (so they can be on your model, just not as a substance designer pack or just a pack of their textures for sale as 'textures'). You need to put a clause (as a text file with the model or on your description, or both) that says:

"One or more textures on this 3D model have been created with photographs from Textures.com. These photographs may not be redistributed by default; please visit www.textures.com for more information."

You can read in more detail here:
https://www.textures.com/contact-terms-of-use.html
6.2

Basically you got it right, but there are few clarifications/corrections to be made.

First of all not all Creative Commons licenses are equal, some of them are non-commercial usage only, some of them are share alike and most of them requires attribution. So that's important thing to keep in mind when searching for Creative Commons licensed textures.

Another thing is specifically about Textures.com textures usage. Actually you are allowed to use their textures for your models and sell those models. Resseling textures, or pack of textures is forbiden of course. There are few additionals requirements and exceptions, so make sure to read full license on the site: https://www.textures.com/faq-license.html#anchor-3dmodel

Every other texture site has their own license, so you need to read them in order to know what's allowed and what's not.

And last thing about CC0 or public domain textures. You can modify/remix them and sell as standalone textures, but modifications needs to be substantial enough for it to be considered different texture. Reselling unmodified textures is not allowed.

thank you both.

in that case i will spend some time retexturing models that i have made earlier.
i have a few original models that i made, but these use texture.com pbr textures so i will have to replace those first. some textures i have to make myself, some i can get from cc0textures.com, but ether way, i have some work to do before i can sell them here.

good luck, look forward to seeing your creations