i can think of fairly elaborate ways that vote selling platforms could anonymise their activity and advertise in discord servers, for example - similar to the way that large pump and dump schemes work.
It would not be possible to track their activity in a totally automated way. This is part of why they are allowed to operate so freely currently.

What people don't seem to understand is that the longer the bots are allowed to run, the more steem gets centralised into the hands of a few bot operators. Those operators can then put more and more of their team into the top 20 witness positions, which then guarantees that the platform will not make any decisions that go against them financially. Currently, the perception that 'bots are good' has many people voting for these witnesses without understanding that it is hurting them.