I thought they had already sold out all those Silk Road bitcoins. If not, that can only mean one thing, i.e. they are selling the same bitcoins twice, thrice, and many more times. "If you give me six lines written by the hand of the most honest of men, I will find something in them which will hang him"...
So it remains widely open how safe (and wise) it is to buy from FBI
http://fortune.com/2016/08/08/government-seized-bitcoin-auction/
Yeah sorry - the post was party tongue in cheek. I should have looked it up to get my facts straight - the auction I was referring to already took place in August.
But no doubt new ones will come up - in different countries. Australian Authorities have done the same thing.