Nowadays many boats are sold through the various self-marketing sites like Craigslists and Kijiji, esp at the lower end of the market.
But not through sneaky 'where can a post this ad?' posts...
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)