How to preserve comic pages drawn by pencil?
There are a number of ways to preserve pencil drawings.  This question is explicit in distinguishing that the drawings were purchased, which introduces another consideration.
A common way to preserve pencil drawings is with a fixative.  How can I protect my pencil &amp; charcoal drawings once complete? goes into recommendations and discussion on that approach.  A fixative works to protect it, but it modifies the surface, and can change the appearance a little.  For acquired works, the fixative would be something the original artist did not apply, and would be an alteration of the work.
If you just want to keep the work for its value to you, protect it any way you like. But if investment/resale value is a consideration, or it's a historical work, your applying a fixative to it could seriously degrade its value.
In this case, a better way to preserve it is to either store it in acid-free, archival quality flat files between layers of glassine paper, or have it professionally framed behind (UV-resistant) glass so it can be displayed and enjoyed.
For completeness, plastic sleeves were also suggested as a means of protection, but Allison C advised that smudging can result.

This is a summary of the comments (primarily by rebusB, Erica, and Gwyn) from within this thread and the linked chat discussion.