Some Poskim imply that a Megillah might be Muktzah every Shabbos of the year. Most Poskim however rule that the Megillah is definitely not Muktzah on the regular Shabbosim of the year and is rather only Muktzah when Purim coincides with Shabbos [in Jerusalem]. Other Poskim rule that the Megillah is never Muktzah, even when Purim coincides with Shabbos. Practically the custom is to be lenient regarding the other Shabbosim of the year however not when Purim coincides with Shabbos.
Shabbos Erev Purim: According to the stringent opinions mentioned above, on Shabbos Erev Purim the Megillah is Muktzah starting from Plag Hamincha.
A Non-Kosher Megillah: According to all opinions a printed Megillah or a non-Kosher Megillah is not Muktzah on Shabbos.
 See Sharreiy Teshuvah 693/2; Sdei Chemed Mareches Purim 5; M”B 688/18; Kitzur SH”A 141/17; Shevach Hamoadim 140; Piskeiy Teshuvos 688/18
The Peri Chadash 688/6 rules that the Megillah is Muktzah on Shabbos being that one is unable to read it on Shabbos due to the decree of Raba that one may come to carry it. It is unclear from the Peri Chadash in whether this prohibition applies to all Shabbosim of the year or only to Shabbos Purim. Aside for this there are Poskim that argue completely on the ruling of the Peri Chadash. Likewise many Poskim that discuss this question do not explicitly explain in their ruling whether they are referring to all Shabbosim of the year or simply Shabbos Purim. This leaves the matter unclear as to whether there is any true dispute amongst the Poskim. [See Sdei Chemed ibid; Shaareiy Teshuvah ibid; Piskeiy Teshuvos ibid footnote 148] The following is an effort to properly summarize this matter.
 Peri Chadash 688/6 as is implied to be understood by some Poskim [See Machatzis Hashekel end of 688; M”B ibid; Shaareiy Teshuvah ibid] See Piskeiy Teshuvos ibid footnote 151 that there is a great reason to say that the Megillah is MMC”K, as it is not used today for any other purpose, other than to read on Purim.
 Peri Chadash 688/6 as is understood by all the following Poskim: Shalmei Chagiga in name of Mateh Yehuda [so is the custom]; Chasam Sofer 195; Chayeh Adam 154/10; Kitzur SH”A 141/17; Sharreiy Teshuvah 693/2; Sdei Chemed ibid; Rebbe in Sichas 13th Adar 1956; Shevach Hamoadim 140; Elya Raba 6881/3 [brought in Shaareiy Teshuvah ibid; Is unclear if his allowance refers to all the Shabbosim of the year with exception to Shabbos Purim, as rules Peri Chadash, or even Shabbos Purim-see Shaareiy Teshuvah ibid]
The reason: As even according to the Peri Chadash ibid the Megillah is only Muktzah when it is forbidden to be moved due to the decree of Raba, and this only applies when Purim falls on Shabbos
 This refers to MMC”K, in which the law is that it cannot be moved for any purpose. However see Piskeiy Teshuvos ibid footnote 149 that some rule it is considered MM”I and hence can be moved for its space or in order to use. According to all one move a Megillah in order to save the Megillah from damage. [Admur 334/12]
 In the Diaspora, in which Purim is celebrated on the 14th, Purim will never fall on Shabbos.
 Machatzis Hashekel end of 688 [however it is unclear if he is truly arguing on Peri Chadash regarding Shabbos Purim; Vetzrauch Iyun on Nitei Gavriel 80/18; and Piskeiy Teshuvos ibid that understand him to be arguing]; Implication of P”M 688 A”A 8; 308 A”A 10; implication of M”B ibid; Shiyurei Kneses Hagedola 308 leaves this matter in question [however is unclear if it refers to every Shabbos or even Shabbos Purim] Elya Raba 6881/3 rules it is not Muktzah however it is unclear if his allowance refers to all the Shabbosim of the year with exception to Shabbos Purim, as rules Peri Chadash, or even Shabbos Purim-see Shaareiy Teshuvah ibid]
 Shaareiy Teshuvah ibid; M”B ibid; Rebbe in Sichas 13th Adar 1956; Shevach Hamoadim 140
 Rebbe in Sichas 13th Adar 1956; Nitei Gavriel 28/6; See Chasam Sofer ibid that understands this in the M”A ibid.
The reason: As beginning from Plag Hamincha one is able to read the Megillah and fulfill his obligation, as rules Michaber 692/4, however when Erev Purim falls on Shabbos one is unable to do so due to the decree of Raba [see M”A 692/6; See Halacha 4A!], and the Megillah is thus Mukztah from Plag Hamincha.
Other Opinions: See Sdei Chemed ibid that explains why the Megillah is not Muktzah from Plag Hamincha even according to the Peri Chadash.