This is unavoidable. There will always be someone willing to burn bridges to gain an extra foothold, always someone that will stab you in the back. As a community, it is easy to advise who cannot be trusted; these people do this knowing they will be branded as such.

There will always be people who have known each other for years. This is an inevitability, there is nothing inherently wrong with this. 

The rampant selling of accounts is not the responsibility of the player, it is on the developer to ensure that they are not disengaging people for a LACK of action. People who sell items/accounts are only looking for a quick buck and are not looking to play long-term and the people who are buying these accounts are not looking to invest in this game. Why invest when you don't have to?

In the current climate it makes sense, people don't have jobs/reduced hours, sell your account and maybe recoup some of your investment. But it does ruin the economy, people's faith in the developer/game and the experience of the game. 

A good example of this is Ark Survival Evolved competitive scene on Xbox. On the 17th of August 2019, for the 1st time ever, the developers dev-wiped all 'cheaters', even if they were cheating to stop themselves being cheated against (To put it simply) This was known as the 'Great Server Wipe' which resulted in all large teams/alliances to lose thousands of hours of their investment - for the betterment of the community. At this time, almost one year on, much of the hardcore community who did exploit quit, but this brought back many, many people who had quit because of the toxicity.

A hard line stance is the only effective solution. 

The take-away? If people provide evidence of people breaking the rules, investigate it and delete/severely sanction anyone involved in selling accounts.