That makes very good sense; what we don't know is how 'sure' Google feel they need to be before devaluing those links.
They seem to have been pretty successful with industrial link sellers - and by extension, their customers (who may not have realised that they are paying for, er, nothing!).
But I wonder how far the algo takes them in the wider world? Google were inviting reports of link selling, which probably accounts for a few more actions (and a lot more comparing their findings with submitted reports!).