It gets worse the more you think about it.  An attacker could acquire coins (if he didn't already have them) by buying private keys which have no coins today but which had significant balances at one point in the past.   Imagine you at one time had 100,000 peercoins but no longer have any.  The private keys are worthless to you but to an attacker which intends to rewrite the chain back from that point they may have some value.  So he gives you 100 PPC equivalent for the private keys which "had" 100,000 PPC.  In this case the attacker isn't attacking for free but he is doing so at very low cost.  He doesn't even need to have already owned or buy up a significant stake and then try to sell them off before the attack he can just attack for millibits on the coin.