First of all not all reputation loans result in negative trust. There is at least one
example of a reputation loan done out in the open and the result was the people speaking out against such reputation loan was removed and/or excluded from the trust list of the person who gave such reputation loan.
The primary difference is that a reputation loan/trade has zero purpose to the person receiving such loan/trade other then to make them appear more reputable. On the other hand if someone has engaged in a number of trades that were mutually beneficial to both parties (of each trade) over time then the person who has earn such trust has value because that trust also comes with a certain level of prestige and will have an easier time conducting (legitimate, non scamming) business.
If someone who already has preexisting value from the trust associated with their account, and they are unable to realize the value from that trust then the chances of that person attempting to scam would increase (as would the chances of them being successful) - this does of course ignore a person's morals/ethics, however this only goes so far as morals/ethics may be ignored if they are desperate enough for money.
You should also look at it from the point of view from the seller: If you have positive trust then there is a good chance that you have made some kind of positive contribution to the community/economy as a whole, so why should this person not be allowed to sell their account while someone who has spammed the forum with useless garbage can receive value for their account? If anything I would think it would be the other way around.
On the other hand when you have a reputation loan/trade, in a best case scenario, the only person receiving any kind of benefit is the person selling such trust/reputation.