I'd say selling to a singular person with a lot of rep is better as all the websites have awful rates. Finding people who regularly buy gold will always be better.
As others have said you just need to buy it from the right person but even then there's no telling if they will get fed up of what they're doing, scam everybody they've sold to in the past and move on with life. Don't get em wrong there are people out there you can buy an account off and never ever have a problem but you'd never really know. It depends what account you're looking for, if it's nothing too serious maybe train it yourself and pay for services for things you hate although, services/powerleveling can be more expensive than buying an account.
trail and error man, people have different experiences. There are a lot of factors, client, script, ip, fresh account, account played legit for a while, bottings times, length of botting session etc.
Tbh I've used op autoclicker for years and garys hood back in the day, never a single ban although it differs for everyone and I can't speak for others
In my experience I think account age matters, I've autoclicked over 500m thieving xpacross accounts, alching back in the day and other things such as the 50k/hr agility a while ago I've had accounts logged in 24/7 for days on end. Never been banned with an auto clicker and this is clicking as fast as possible without breaks. Difference from a script I'd say is the client being detected