which sites have information about the details of every set?

[ks6349]

I mean, for example, how many bags are contained in a set and what they are correspondingly. Since I have some sets that I don't like parts of them so I'd like

to sell them away but I do not want to unopen any bags because it will not be regarded

as new and final value will decrease

[icm]

If you want to know what sections of a set are in which bags, the best thing to do is to download the PDF instructions from Lego as linked through Brickset.  You can also look for speed build videos on Youtube; they usually show how many bags are in the box and often mark when one bag ends and another bag begins in the build.

[Lego David]

Brickset and Bricklink are the best in this regard. Of course, they don't have all the information regarding every set, but their database is the best we have, and they have successfully archived pretty much every single LEGO Set ever made (even if information regarding old or obscure sets may be scarcer).

[TeriXeri]

Rebrickable.com has a visual overview of parts in a set as well, and you can compare it to the rest of the collection, it's not as simple to use with a large collection however (but it has options to filter out built sets or loose parts), but can synchronize with Brickset.