Ignoring whether or not this is against the TOS since there seems to be some disagreement over whether it is or not - there are a bunch of reasons you'd want to be extra careful if you did something like this.
Firstly, it would be a huge mistake to purchase a channel that wasn't making the same kind of videos as you plan to make. If you bought a channel with 50,000 subscribers, that was say a music channel.. and you started posting gaming videos. Guess what, those subscribers aren't going to be interested. So now you just have a bunch of dead subscribers, and you're no better off than someone who did something as dumb as sub4sub.
Now let's say that you buy a channel in the same genre. Well now you also have a problem: The fans. People who were genuine fans of the person before you who owned the channel will likely be up in arms about the new ownership. Dislike brigading videos, mass unsubscribing, and simply not watching your videos in the first place. If you really wanted to avoid this, the person owning the channel would really have to introduce you gradually. Get you featured on the channel from time to time long before the switch was made, so that their viewer base was familiar with and already liked you before the transition. Even then, even if you did that, a significant percentage of the subscribers would no longer be interested in you, because certain things about the original owner that made them subscribe are no longer there with your version of the channel.
Then there's the fact that you haven't learned anything. It's all very well having a bunch of subscribers for free, but if you handed over say PewDiePie's channel to the average person who's just started YouTube, they'd run that channel into the ground in under a month. There are certain things about growing and maintaining a channel that you only learn through direct experience and through doing those things yourself. So if you just take on what someone else has already built, you aren't going to be able to really continue with it very easily. You would really need the person to sit you down over several days and teach you step by step their entire process: from editing, to SEO, to external advertising etc.
It's why a lot of people born into wealthy families end up being unsuccessful despite all the advantages they had. They
had money from the get go so they didn't have any real reason to learn how to make it. As a result they never learnt how to be successful, and just mooched for as long as they could until that was no longer an option and they had to get a regular job. On the contrary, a lot of the most successful and wealthy people had very humble beginnings - because to grow even a little when you start off with nothing, takes a lot of hard work, so you really learn how to be successful along the way. These same people can even lose all their millions/billions, only to have most/all of it back a few years later - because they learned the strategies along the way.
On the other hand, let someone win the lottery, and sure they might have $10M.. but what happens once they've spent it? They don't get it back, because they never learned how to get there. It was just given to them for free with no instructions.
Same thing applies here. What happens when the new channel you buy runs into issues that you've never faced because you've never grown a channel to that size before? What do you do?
That's not to say that it can't work, but I just don't think it's a good investment, unless you're say getting a channel with like 100,000 subscribers for pennies. I wouldn't do it, even if TOS does allow it.
As
@TwoTakes said: Better to spend that money on things that will allow you to make higher quality videos. Better recording equipment and lighting. Editing software. Other software like say VideoScribe if you're that sort of channel. Hire someone to do annotations or even someone to help you learn SEO etc.
