I agree with your calculation except for the last term, which I would have said is $$ e_n^*\wedge e^*_2\wedge\cdots\wedge e^*_{n-1}\wedge e_1^*. $$ Note that each term except the first one in your calculation differs from $e_1^*\wedge\cdots\wedge e_n^*$ by a single transposition, so you get $$e_1^*\wedge\cdots\wedge e_n^*-e_1^*\wedge\cdots\wedge e_n^*-\cdots-e_1^*\wedge\cdots\wedge e_n^*=??$$