Sure there are instances where people will sell their accounts plus their private keys however trying to scam with a trusted account is a gamble at best and a gamble with small potential rewards and high risk and a generally small success rate. I would call the EV of buying a trusted and/or a default trust account to be negative by a long stretch.
It's impossible to calculate without real, actual, values. The value of the default trust account to someone who's going to be walking away from the forums forever (for any imaginable reason) independently of whether they find a buyer is whatever they can get for it. Ie, if you know you'll never use it again, then you've got every incentive to sell for whatever you find someone willing to offer.
It's impossible to say what the value of something is/isn't without knowing actual details of a given situation. Anyone who says otherwise is engaging in mere speculation---this can be fun, but it's not solid argumentation.
I know what people are generally willing to pay for default trust accounts, how much default trust accounts have sold for in the past, I know how much people are generally able to hold for a third party before suspicions start getting tripped.
It is pretty rare that someone attempts to use a purchased account to scam many people for as much as possible however in almost every case that I have seen the amount scammed was significantly less then the value of the account and in cases when the amount scammed was greater then the value of the account, it was because one person who doesn't know how to take proper precautions got scammed for a large amount.