Actually, these bitcoins are ones that Ulbricht claims are his, not the (much larger) set of bitcoins that weren't on his personal hardware which were linked to DPR which he did not claim (and which were the subject of an earlier sale.)

Oh, my bad. Sorry. I thought this was selling off more of the earlier Silk Road stash, not the "personal collection". Should have read more carefully.

How does he claim to have acquired them, or has he not made any statement on that yet?

When anything you say may be used against you, would you make any such public statement before the matter comes up in court?

In the agreement document he explicitly claims ownership, and agrees to the sale since the price may go down before they are returned to him.