Why should it stop account sales? The majority of people involved in this kind of business know the rules that a red trust is the logical consequence for account sales. They will continue it whatever we will write in the rules. The only question would be if they are all reading the rules and if there are some users who we want to be protected from red tags
At least account sellers are well informed that account sales are discouraged and that caught accounts will get a red tag immediately, that's their business. And they won't care about any rules as long as they can make money and sell their hacked / stolen accounts.
I don't have any clue if account buyers are reading the rules, let's assume Coolcryptovator is right and there are different incentives to buy accounts:


I'm going to add a little bit to your conclusion:

1.1  Newbies (get quick rich) who want to bypass the Merit system and earn more money by high-ranked accounts for shitposting: Maybe they read the rules but I doubt it
1.2 "Shitposting agencys" using dozens of high-ranked accounts to bypass the merit system and shitpost the forum for signaure campaigns: I think they know the rules and red trust
1.3 Red tagged users who want to continue earning with fresh account (mostly a mix out of 1.1 and 1.2): Well, they should know it for obvious reasons.
2.1 Scammers using their high-ranked accounts to deceive and scam inexperienced users: I'm sure they know that account sales will result in red trust

Honestly, the only party I would halfway like to protect are 1.1 Newbies if they are scammed by account sellers. The rest are all deserving the red tags and wasted money for their tagged accounts.

Where did you read that it's explicitly allowed? It's allowed, but account sales are discouraged. That's a big difference. In the same sentence it's clearly mentioned that account sales are discouraged. Why should someone do something discouraged "on their own risk"? It's like "It's allowed to jump from the bridge but it's discouraged".
Maybe we can add that there could be consequences if someone is engaging in account sales: and DT will possibly give a red tag for account sales because it's untrustworthy.

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I don't see any other (legit) reason to buy a high-ranked account than the (shady) reasons I've mentioned above. Normal forum users can do everything with a Jr. Member account or buy a Copper membership. Account sales are mostly related to any kind of shady business. @Quickseller: If you have a suggestion to legalize account sales while it's ensured to exclude the types of shady activities I've listed in my post I'm sure the community is open to discuss it. But I don't see how this can be ensured.

To make it short: Yes, red tags from DT are totally appropriate for account sales in my opinion because we all know what would happen if we stop tagging account sales. Red trust is given to them because red-trusted accounts can't be used anymore for the (shady) reasons why they were bought. No need to ban them.
I'm sure the community is open to debate how the inofficial rule No.18 is written down to make it clearer for inexperienced Newbies that account sales are allowed but there will be negative consequences to engage in them. I remember it was written down like that in the inofficial rules because account sales will be done just outside of the forum and we can't stop them, that's why it's written there as "allowed but discouraged".