40 CFR 90.206 - Trading.
(a) An engine manufacturer may exchange emission credits with other engine manufacturers in trading, subject to the trading restriction specified in § 90.207(c)(2).
(b) Credits for trading can be obtained from credits banked in previous model years or credits generated during the model year of the trading transaction.
(c) Traded credits can be used for averaging, banking, or further trading transactions, subject to § 90.205(a).
(d) Traded credits are subject to the limitations on use for past model years, as set forth in § 90.204(c).
(e) In the event of a negative credit balance resulting from a transaction, both the buyer and the seller are liable, except in cases involving fraud. Certificates of all engine families participating in a negative trade may be voided ab initio pursuant to § 90.123.