We all want to believe that people selling horses are honest and forthright, but that may not always be the case. As for drugging, there are drugs that are effective for 10 and even 30 days, so I would wonder if the horse I see on the internet, or in person, will be the same horse I get home.
Require as part of the PPE, blood to be drawn, and ask the vet to check for any substances that could have been administered.
I have heard horror stories of people buying a horse that is "bombproof, beginner safe, anyone can ride..." and the video shows mounting, dismounting, standing on, bullhorns being blown while in saddle, all the things someone is looking for in that calm, quiet horse...only to get the horse home and have him/her be quite the opposite. I know of 3 people in the past couple of months who had this experience & have had to return the horses.
While it takes a period of adjustment for any horse to become comfortable with new stable mates and humans, the horse represented should be the horse purchased. Good luck in finding that perfect fit!