CNN — For years, Tesla has earned billions of dollars from its competitors just for selling electric vehicles. But that windfall is about to go away, just when the company may need it the most. Regulatory credit sales have been a huge source of revenue for the automaker, which currently faces a sales and profit slump. Legacy automakers purchase credits from Tesla to keep selling gas-burning cars that would otherwise violate emission regulations and cost them a fine. But the Republican tax and sp...