They are most likely barter transactions, probably amongst corporate mates. They enter different industries on the planet with an agreement to exchange goods to support each other’s operations. Or they had a neighbor bring goods out from the exchange planet in what would have been otherwise empty cargo space.
They could also be parts of multi-contract shipping deals…where the payout to shipping party takes place in a different LM ad. Some people do their shipping contracts as three contracts — two contracts for 1 monetary unit, then a third contract where the shipper appears to dramatically overpay for a single unit of a common good like DW.
So, bottom line…these could absolutely be cheating. But they could also be part of a perfectly legitimate much more complicated transaction that the current limitations of the contract system don’t allow to be directly represented. The shipping update will likely reduce the frequency of these agreements, but I think they will still occur to a limited extent.