Although Sentinel isn't 100% accurate, there are still other ways to make sure if someone was using illegal modifications or not. I'm mainly speaking about Sentinel because mods are trained and have the ability to see the difference between hacks and good Pvp, so if someone got banned by a mod, then there's a pretty small chance the appeal wil get accepted even though you might have a rank.
Ranks are splitted seperately with the ban system. It's not because you bought a rank you get immediately unbanned. You were now talking about buying an unban. It seems in my opinion a bit like scam. You (players) buy for something that in most cases gets removed after 7 or 30 days if we speak about illegal modifications appeals. I don't know why you want to lose your money by buying an unban on a minecraft server, that sounds pretty stupid.
Maybe 95% of the players (this is a fake number ofcourse we don't have official statistics of that) that get punished for breaking a rule, were actually breaking a rule. So if every player buys an unban, they can keep hacking over and over again and it means more work, because we already have to deal with alts. Ofcourse we get a lot of money for that, but we probably are going to lose players due to that. The amount of hackers will raise because they can buy an unban and the amount of fair players will decrease because we became a server full of hackers.
I'm fairly sure buying an unban won't become a thing, as it looks a bit like scamming, it almost seems impossible to become a thing and The Eula...