Vote-buying is already criminalized under 18 U.S.C. § 597 [0]. Several states have similar laws [1].

Scenario A could still happen, but seeing as it's explicitly illegal, the likelihood of it causing widespread voter fraud is unlikely.

[0] https://www.law.cornell.edu/uscode/text/18/597

[1] http://www.lrc.ky.gov/Statutes/statute.aspx?id=27347