The problem is that the site will lose affiliation with 37signals (which probably drives a lot of their traffic) and existing customers will have to renew their membership. It's safe to say that when buying it revenue will be significantly lower than what it's doing right now.
So the vast majority of this site's value is actually its relationship with 37signals? And 37signals wants to sell the site and dissolve that relationship?
The sale price ($480k) is just greater than twice 1 year of income ($212k). Why does this seem low enough to be a red flag?