That's not possible to opt in to under the GDPR, FAFAIK. People can consent to the purpose for which their data will be used, and any such purpose must be enumerated explicitly. "Selling the data" is not a processing purpose, and it would still be illegal even if the person consented (it does not meet the bar for informed consent).

Under the GDPR, the only legal way for personal data to be transferred between companies would be for the "buyer" (processor) to use the data original data from the "seller" (controller) on behalf of the controller. The data, as consented to be collected, remains the liability of the controller for its entire lifetime.

GDPR is not really about selling personal data but about using it; a big part of its effect is on buyers of data since GDPR effectively (there are all kinds of nuances) means that it's not legally possible for legitimate company to simply buy personal data for arbitrary purposes, since the data subject obviously did not opt-in to that particular purpose by that company when the data was collected.