these limitations you are proposing will actually make things a lot worse.
for example if someone was earning $10 and you limit it to $5 then they will create 2 accounts and now they have to spam twice as much. if you ban account buys it will just migrate to somewhere else and because of that ban (less supply) the price of such accounts shoot up so there will be more incentive to do account sells and more people would start doing it (farming accounts and selling them). the result is more spam, more merit abuse,...