I don’t think it’s so much a concern about violating the ToS as it is the consequences of violating it. Valve may be able to lock the account or close it if they have good reason to suspect the account has a new owner. An example of solid evidence of this may be changing the payment method for purchases, such that the name on a credit card doesn’t match the previous cardholder name on the steam account.
But if the new owner doesn’t plan on making new purchases on the account, it would probably be more difficult to confirm the account was transferred.