It's unlikely that he sold his coins since they haven't been moved on the blockchain. The only way he could have sold them while still remaining undetected would be if he sold the private keys to these addresses instead.
Goodness; why in the world would someone buy private keys??? The original owner could still have them and move the funds before the buyer. The only way I would agree to buy private keys would be if the original owner sat there while I moved the funds.
Because they are not traceable through the block chain. Every bitcoin you spend gets linked through the block chain to where you got it, and usually to other coins you had in your wallet at the time and where you got them. It's pretty easy to analyze how you use bitcoin and see your past transactions, if you start with a single address of known ownership.
But if you buy a private key from someone, the coins do not move on the block chain. If they are freshly mined coins, they have no history to link to anything whatsoever. So they can be spent, carefully, without being linked to any of your previous or future purchases or spends on the block chain.
The buyer immediately transfers the coins to his own private key so that the seller can't spend them, but as far as block chain analysis can tell, it's still not linked to anything else the buyer has.
