Since voting by mail is already available to anyone in any state, this widespread vote-selling should already occur, no? "Send my $100 and I'll send you my unm

Since voting by mail is already available to anyone in any state, this widespread vote-selling should already occur, no? "Send my $100 and I'll send you my unmarked absentee ballot for the 2012 elections - you can fill it out and mail it in." But it doesn't happen. At all. Possibly because it's illegal.