Doing scalable business isn't 37signals' strength. And selling, networking and dealmaking neither => Flippa is generally for smaller sites/SEO stuff for low prices. This size of sale is better done through contacts or just a good seller who has these contacts and gets a commission. Otherwise you risk reputation damage and external effects on your other brands and entire group if offering your products/companies directly on Flippa or eBay (which feels just desperate).
Anyway, they had to raise funds and put a dedicated team on it when Sortfolio/Haystack was at its peak. Now, it's too late looking at Sortfolios decreasing traffic on Alexa. Nobody will buy it and that's something DHH never will get: it's not about your current revenues or status, it's always about you future expectation when valuing assets/companies/startups/businesses and that's how you sell