In my opinion, it is all about who botted the account. If the guy has experience in botting accounts, he will know what methods to use and what to avoid to make sure Riot doesn't detect botting on the account.
The ban waves are really random with my experience but if the account was to get banned for botting, it'd be banned within 2-3 weeks at least.
If the account has survived more than 1-2 months of ban waves, it clearly indicates the account was botted by an experienced botter, the only hurdle I'd say is getting reported by a salty opponent that have manually search you up on maybe lolnexus.com or check your profile and reports you for having a botted account and eventually getting banned.
I was selling IP smurfs on eBay by purchasing them off numerous bulk sellers and selling it for a little profit. The bans were very patterned as I had contact with at least 5-8 bulk sellers and with some of them I had 0% ban rate while with others I had nearly up to 100% ban rates. I would really advice you to have a look at trusted sellers with 14 days+ refund/replacement policies to enhance on being safe. Wait till 2-3 weeks to be kind of safe to make costly investments on your accounts.
This is an opinion of mine with experience on botted accounts, I still use my botted accounts from year or two ago and they're working completely fine. Just be sure who to purchase it from and you'll be safe. Hope I helped you and have a good day