A common theme among the people who are voicing their opinion against the sale of forum accounts is that the sale of accounts leads to scams. I would however question that conclusion and ask how many scams (both as a percentage of account sales and as a percentage of forum scams) are from sold accounts and how many of those were made possible (or at least aided) by the fact that it was a sold account. I would believe for that number to actually be quite low and in the grand scheme of things, allowing the sale of accounts probably actually decreases overall scamming activity because it gives people who are desperate for money one additional way to get money that does not involve stealing from others.
Some of the accounts that were involved in scams were sold accounts (by checking their activity, password status and posting style). Recently I saw two sold accounts scamming users in the Currency exchange section and they were accounts not used since 2012/2013 with one having a positive trust while the other one dint have any trust and the rank was low. Since the accounts were old, the person sent first and then got scammed. The percentage may be low as some of the accounts which are sold can't even be detected while some can. However, I don't believe that account sales have reduced these scam activities when instead they have increased chances of getting scammed.