If a person wants to make a fraud he can open a new account in person. But we have to wait for our rank to increase in order to be able to join signature campaigns.
Only if people trusted newbie accounts! It's hard to farm an account just for signature campaigns as one can easily make out if the account bought.However the case is not the same if a trade is involved with the bought account.People tend to fall for the account ranks or the trust feedbacks.
Yes, people do not trust a newly registered person. But it is known by the forum when a very high quality account is changed hands.
There are also people who want to earn BTC from their signature campaigns. This requires a high-ranking account. Not many BTCs are earned in low rank memberships.
So every account is not made for shopping fraud.