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 Pasuk19) that Pharaoh would initially not listen to him. 1
Hashem had already told Moshe (see earlier, Shemos 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 #4: What Moshe meant was that they would not believe that he had been sent with the Name of Havayah (the Name of mercy by which Hashem would perform open miracles). 5