Sulfate dianion is a very common counterion. Here we have sulfur in a VI+ oxidation state. For "thiosulfate ion", ONE of the oxygens has been replaced by a sulfur. And in the replacement, sulfur has assumed PRECISELY the same oxidation state (-II) of the oxygen it replaces (and such a formalism is reasonable, because sulfur and oxygen are in the same group!). The average oxidation of both sulfurs is still +II, but we can individually assign separate oxidation states to the central and terminal sulfurs.
Both sulfate and thiosulfate ions have resonance structures, typically ""^(-)O-S(=O)_2-O^-, and ""^(-)O-S(=O)_2-S^-. Of course, the negative charges are delocalized around the chalcogen atoms.
