If you send money from your account to Minnowbooster it's visible, of course, or if you withdraw money from there. But otherwise you can use their webpage to purchase the votes: you just enter the post url on Minnowbooster page, so you don't need to give the post url in a blockchain transaction.

Also back in the days when Randowhale was the only vote selling bot, people used to buy more votes for friends and great posts, if they were unable to give high votes themselves and wanted the post to get a good payout. So I'm assuming people could still use purchased votes to support friends, as it's easier to spend some SBD to buy a vote for a friend than get enough SP to give it a big vote.

But on the third point, Minnowbooster is also using the "vote seller" accounts. If I would purchase a vote from minnowbooster, it might not be minnowbooster account who upvotes me. Instead they're using bunch of accounts who have given permission to sell votes to upvote them.

Like I'm currently having the vote selling on for MB whitelisted users. If I'm having more than 90% VP (which never happens when I'm active), my votes can be used for vote selling. So this would work only on times when I'm completely inactive for few days, when I'm manually upvoting I'm usually around 60-80% on VP.