It's trackable.
Read minnowbooster/smartmarket wallet for transfers of money with a website link.
Cross reference timing of upvotes on person's account by people who allow those services access to post on their behalf.
these services usual return funds not spent on upvotes. so you have a window of voting, on a specific article and have a list of people known to sell their vote.
Until they change the method... this will work.
only if they transfer the SBD from the Steemian wallet to the minnow booster / smart steem wallet, if they use the internal SBD accumulated, there is no wallet memo..... not that I can see any
even if there is a link to the memo from the SBD, how would you correlate lets say my upvote done at the same time, it could be genuine, how would you know it's part of the bid upvote, there is no true timing correlation, they could use 2 accounts to give $10 or they could use 200 accounts to give $10, how would you differentiate on a popular post with 1000 different accounts upvoting
this is how the yel'pap does it he uses the shill minnow upvotes and let the view rackup from trending viewers who won't upvote to justify making it look instantly popular when it's really majority paid upvotes
i did appreciate your feedback about using that timing and memo, it's tricky to catch all these creative ways of bidding