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Someone who wishes to leave negative trust today for someone who scammed someone two months ago should check the security log for any password changes between when the scam occurred and today. If the password changed in the relevant timeframe then someone wishing to leave negative trust needs to consider if these password changes reasonably resemble the ownership of the account changing hands. If the password has changed between when the scam occurred and today then ownership of the account changing can be refuted with blockchain evidence (and/or other evidence to otherwise prove that the person before the password changed is the same person as after the password changed).
In the same example as above, if someone scammed two months ago and I leave negative trust today, if the password were to change in a week I would not consider to remove my negative rating just because there is evidence that ownership changed hands after the account was marked as a scammer.
#1 use PayPal for a deal
#2 wait 1-3 months
#3 fake account trade
#4 wait a little longer
#5 charge back
#6 "account was sold m'kay"
#7 profit
A couple of points.
#It is a bad idea to accept PayPal from someone unless they have a very strong reputation, and this is regardless of the amount in question (it really is an overall bad idea to accept PayPal at all).
#Accounts with very strong reputation have incentives to maintain their reputation because of the money they can earn from such reputation (ex., from escrow fees, from being able to charge a premium/pay a discount for Bitcoin and other things that are traded, borrowers are more likely to request loans from reputable lenders, being able to charge a premium for their signature, ability to run a business).
#someone who is able to buy any significant amounts of Bitcoin with their PayPal will have significant amounts of trust that would equate to people trusting them with large amounts of Bitcoin/money either via sending first or trusting them to act as escrow.
#when an account is revealed as being sold, it will often lose significant amounts of its trust (ratings).
#The prices of forum accounts are currently very low.
With the above being said, if someone were to (attempt to) charge back PayPal transaction(s) worth large amounts then they might as well simply take Bitcoin (and/or other valuable property) without giving their trading partner anything in return. If they were to try the scheme you described then they would likely lose their existing reputation and would be left with an account that they might as well purchase on the cheap.
Also, PayPal does not have a "chargeback button" and they will not necessarily agree with every dispute, especially those regarding transactions that happened several months ago.
There is also the point that the admins can better tell for sure if an account was
actually sold and can leave trust accordingly if they feel that a negative rating is warranted.