Buying and selling of accounts facilitate spam, shitposting and account farming. They are marked with a red tag in case they are caught. Sold accounts are the biggest source of scams happening and then the shiposters are always there.

The easy way to understand why its discouraged/marked red is that those people whose accounts have been sold had some trades with other people. The trades may have built a good rapport among them. The new owner might take advantage of that socially good terms to implement a scamming method.

BTW if you can get an info on which account is being sold and who is buying it report it in "Reputation".