Given the prospect of a severe revenue dip once Sortfolio is detached from the 37 signals brand, the asking price (450,000) is very high. Moreover, as far as I can tell, they didn't respond to bidder questions on flippa. To me, it seems like the bidders and the auction were ancillary to their actual intent. Like other posters, I think their actual intent was to gain publicity for the 37Signals brand (not Sortfolio).
I expect they will follow through on their original pronouncement by shutting down Sortfolio soon; however, I don't think they ever had any intention to sell it. I think they are smart enough to know that someone with 450,000 in cash is also probably smart enough to know that it was a bad deal. By providing a spectacle by way of flippa, the narrative is now more impressive, they reaped a lot of positive press, and they reinforced their commitment to their core products.
It's no surprise nobody bid. Actually, someone did bid but nobody at 37Signals approved it, so the bid expired. You have to wonder why they spent money listing it at all; and, since the auction was essentially abandoned, whether they would've honored a sale if someone had offered the asking price.