I was thinking about buying assets, but I don't want to waste time and want to acquire assets as fast as possible, and acquire a large amount of them, because I want to make several different games from the assets (rpg, action, shooter, etc.) Also, I want to commercialize the game, so what would you recommend me doing?
I'm not sure if this is on topic, but I'll try to answer anyway: You don't need to pay for your game's assets; That's why things like the Unity Assets store, ue4 store etc.. exist ;)
When it comes to commercializing the game, you really want to check for your assets' licence/s (unless you created them yourself of course), because they tell you if and how you can use them in your game.
If the assets are not free for commercial purposes, you probably need to pay the artist who created them (again check their licence requirements).
If you don't want to create the assets yourself, there are a whole lot of websites that you can find with a simple search on the web, but for a bit of money you can pay an artist that will make exactly what you need, exactly how you want; That's the main advantage.