Being able to sell and buy votes is a good thing, it allows people to compromise. Now it doesn't work because a vote is a rare resource which cannot be priced, but when there are frequent votes on individual issues, you can think of a monetary price for it, or support one law in exchange for someone else supporting a law you care about. Today when politician promises to raise some taxes and reduce others, it is also a method of buying votes, but indirect and prone to fraud.

In Rome there was a clear hierarchy of patrons that was much more influential than what we have in today's society. Secret ballot broke it, and the result was a disaster.

Take a look at https://voteflux.org which explains the idea better than i do. To pass a law you need a large majority to support it, and the rest to accept it enough to not go to constitutional court. And if you are able to bribe such a large number of people continously, it means that they get more profit from the law than from repealing it, so your law is good for society.