Why would they have a hard time spending them?
Because we can track which addresses he's sending them to.
He would need to mix those coins really well... Or send it to some online wallet (HA!! no pun intended here) and withdraw again.
We have no idea where the money is. People seem to be ignoring the crux of the issue here.
What we can track is just the 4100 BTC that TF reported stolen. We can argue all day about whether or not that money was really stolen, but that's besides the point.
Here's the issue: CoinLenders alone had at least 10,000 BTC. Inputs, CoinLenders and BTCINVEST combined, probably held at least 20,000 BTC. Maybe it was much more, we have no way of knowing. All we know is what was reported on the forums, but there could be a lot of people out there who had money with TF and never wrote anything on these forums.
We know that only a tiny fraction was returned after the hack. Where is the rest? How much is the rest? How much money is actually missing? We have no idea, but clearly the 4100 BTC that were or were not stolen are a very small part of what TF still owes.
The remaining money is probably split over multiple addresses. Much of it probably doesn't even exist in the form of money, but as debt owed to CoinLenders, ASICMiner shares, JustDice deposits, and other such assets.
Most of these investments lost much of their value lately, which is why CoinLenders was almost certainly on the verge of bankruptcy even before the alleged hack. But they are still worth something, and we will probably never see a dime of it.
That's the real money we should be focusing on, and no, we cannot track it.
