Why did Moses think the Israelites would not believe him when he returned to Egypt?
Moses reasoned the Israelites would not believe him because he had been absent forty years and was returning as an old, lowly shepherd. He recalled that even when he had Egypt's pomp and circumstance, they rejected him. He expected them to say the Lord had not appeared to him.
Les explains that Moses responded with "but" just as believers often do when God calls them to a task. Moses had been gone forty years on the backside of the desert as a shepherd, so he feared the Israelites would not believe his report. He remembered that when he came with all the pomp and circumstance of Egypt and thought he could have had everything going for him, they still did not believe him. Now returning as an old lowly shepherd, he doubted they would accept him. Moses finished his objection by saying they would claim the Lord had not appeared to him. God then asked what was in his hand, the rod, and told him to cast it on the ground, where it became a serpent, and Moses fled because Middle Eastern serpents can be deadly poisonous.
And Moses answered and said, But, behold, they will not believe me, nor hearken unto my voice: for they will say, The LORD hath not appeared unto thee.