Two men enter the shop and start speaking with me. When they leave the saleswoman asks me: What they wanted?
I answer: "They thought I was someone else".
How I translate this:
Они думали что я (был ) кто-то другой.

Also how I translate: There is a possibility that the were searhing for someone stupid.
"Есть возможность (или наверно или вероятно), что (.) они искали какого-то идиота".