And the politician cannot sell if he doesn't know who the buyer is! Simply permit any and all donations to party or candidate, but require that the source remain anonymous. Make it a felony to disclose the source of contributions. Take one of the existing useless federal agencies and assign it the duty of collecting and disbursing the funds.
That permits individuals, corporations, unions etc. to support parties or candidates as they see fit, in keeping with the spirit of the first amendment, without allowing them to buy influence.
Are you crazy!
The exact opposite is needed.
Full disclosure of who got what from whom.