You really think the FBI talked to DPR and discussed with him whether now would be a good time to sell? They sell now because they can, the FBI are no speculators and they certainly didn't ask for DPR's permission first.
Actually they did talk with Ross (not clear on whose initiative) and both agreeed to sell. The FBI still cannot sell those coins on their own, it is not yet decided whether they belong to Ross or to SilkRoad. If they belong to Ross, they would have to wait for the verdict on Ross's trial.