Thing is you don't want to sell to a nontechnical owner. I've sold apps in the past where it all just died, because the purchaser did not put in any effort.

You want to sell to a party who will run it as if it was their own product (or who will hire a capable team to expand it further). That is possible for a $400k ARR product, but much harder for a website doing say $50k through one-off sales.