They get banned because they are scamming. Paypal account sellers are supposed to provide what they advertise and it's usually because of that that they end up banned. If your account gets limited after sellers advertised warranty period then it's not their fault, but usually they deliver faulty products to begin with or end up being at fault one way or the other as creating quality paypal accounts is a delicate process.
One of the reasons why there are many banned paypal account sellers is probably because it's more "profitable" for them to exit scam with paypal accounts as they can cost up to 200/300 especially if bundled with ebay.
If you are afraid of getting scammed,you can always use a middleman which can hold the money until account is delivered. Additionally feel free to contact me or any of the marketplace staff if you believe someone needs to be additionally checked for being a previous scammer/ban evader. @Jaladhjin