What made Moshe think that Bnei Yisrael would accuse him of not having been sent by Hashem?
Ramban and Seforno: Because Hashem had just told him (in 3:19) that Pharaoh would initially not listen to him. 1
Oznayim la'Torah: Because he had a serious speech defect, 2 and they would not believe that Hashem would pick as His Shali'ach someone who was a poor orator. 3
Seforno: Since "whatever He says comes to pass" (Tehilim 33:9). Also refer to 4:1:2:3.
R. Bachye: He was unable to pronounce certain letters.
Oznayim la'Torah: Like we find by the Navi Amos, who stammered.
Hashem had already told Moshe (see above, 3:18) that Bnei Yisrael would listen to him. Why did Moshe say that they would not?
Ramban #1 (citing the Midrash): Moshe's query was indeed unjustifiable in light of the earlier Pasuk. 1
Ramban #2: The earlier statement was not a prophecy, it was a command - that they should listen to him (and not that they would necessarily do so). 2
Ramban #3: Hashem told Moshe that the elders would listen to him and accompany him to Pharaoh. 3 Moshe was referring to Pharaoh's harsh reaction, 4 after which Bnei Yisrael were likely to reject Moshe's mission completely.
Ramban #4: What Moshe meant was that they would not believe that he had been sent with the Name of Havayah (the Name denoting Chesed by which Hashem would perform open miracles). 5
Gur Aryeh (to 3:18): Hashem had assured him only that they wouldn't dismiss his words out of hand, without even considering them (for he would use the key words, 'Pakod Pakadti'). This did not guarantee that they would believe and accept the content of his message; and this was Moshe's concern here.
