Sounds like the seller's realtor did an excellent job representing them and landing the sale. I'll wager the buyers were represented by an agent as well, since it cost them nothing to use one.
In contrast, its very doubtful the average homeowner would be able to close a deal like that on the spot even if they had been available to engage with some random buyers on a moments notice. Salesmanship is a very underrated skill.
That's $18,000 (6% commission) the seller had to pay whereas if they did FSOB for a few weeks they'd saved that money.
I've sold real estate privately before, but there was always a broker involved if for no other reason than it ensured compliance will all state/county/local regulations and removed substantial liability risk from my shoulders. Just type FABO+lawsuit into any search engine for more infomation.