“We sell sandwiches in/at our/the shop.”
Which is the right preposition for this sentence?

All those are possibilities.

Both “in” and “at” are possible. Strictly, “in” means “inside the building”, while “at” means “at the location of”, but in practice there is little difference in meaning in this case.

Compare “We sell sandwiches in/at our stall”, where “in” sounds odd since stalls do not have an “inside”.