Account sales are very difficult to prove. It is possible to have some suspicion than an account has changed hands but no way to know if there was any monetary transfer. Proving your suspicion is also very difficult to do, so it may be for the best that the mods stay out of it.
The sale itself might be difficult to prove, but that's different from having sufficient evidence that an account has changed hands such that red-tagging the account in question is justified. I spent a
lot of time between 2016-18 sniffing out hacked/sold accounts, and you could ask a lot of other members here as well how easy it is to show that an account changed hands--large gaps in posting followed by a huge change in writing style is a big giveaway, and there are others too.
Could we have a standard that cases of account sales get a red tag? I see it mostly attracts neutral tags now and no one can decide what another does with their trust ratings, but if it gets a negative tag, it will make it less likely those users are trusted for financial deals.
I went on a rampage during the time frame I referenced above, red-tagging anyone who even showed an interest in buying/selling an account. Then I got made a DT2 member and was kindly asked to remove a lot of those negatives. However, Theymos has said tagging buyers/sellers is a legitimate use of the trust system (he made a post in reply to one of mine, but damned if I can find it). As far as making it standard....that isn't going to happen, because even among DT members there are mixed opinions as to account sales and whether they're so detrimental to the forum that action should be taken against sellers and/or buyers.