FBI can't legally dump anything until the whole case goes through the court system.
then it will be publicly auctioned at market prices (not sold with slippage). fbi etc also wan'ts to maximize their returns.
Still haven't seen the legal argument as to why the feds can't file for civil forfeiture in parallel to the criminal case. It is not out of the ordinary. If they did that, they don't have to wait for conviction. If Ulbricht can't win an "innocent owner defense", they can has all the coins.
In theory they "can" in reality they "won't".
Remember asset forfeiture may seem like a big deal to us but it is routine for the feds. If they win the criminal trial the forfeiture is a done deal. Prosecuting two cases means splitting resources or running two teams. Even the DOJ (and it is the DOJ not the FBI who will prosecute) has limited resources. They likely have a backlog of hundreds of criminal and civil cases. Lastly prosecutors don't want to give away any of their game plan. Having the forfeiture trial first allows the defense team a "do over" on the criminal trial. They get to see the prosecutor's play book and adjust their response. Those are the big ones but there are other minor reasons as well. It just looks bad in the court of public opinion "stealing the man's money before he is even convicted, whatever happened to justice blah blah blah". Even if the prosecution tried the defense likely would file a motion stating they lack the resources to handle both cases and the same time requesting one or the other be continued.
So yes they "can" do the forfeiture first, during, or after the criminal trial but in reality they will wait for him to be convicted just like they do for thousands upon thousands of other criminal cases every year.
Also not sure why people think the SOVEREIGN NATION OF THESE UNITED STATES (bold for emphasis and officially sounding presentation) is going to use a Bitcoin exchange. Forfeiture auctions don't work that way. The federal government isn't going to take counterparty risk. They will post a date for the auction and if interested YOU wire funds as a deposit directly to the treasury IN ADVANCE. Don't want to pay by bank wire, don't want to pay in advance. Tough you don't get to participate. Remember this is a government (arguably one of the most powerful on the planet). They make the rules and you follow them or you don't play. The government isn't going to put $20M on MtGox or BTC-e (where the fuck is BTC-e located east bumfuckistan?) to have it "hacked" and stolen from them.
Never going to happen not in a million years.