Like most people I'm also a beginner and my understanding of the game is still quite shallow but as someone who bought 100+ packs here are some of my thoughts.

....if i am a new user and i'm about to built my deck why should i purchase a booster-pack which will cost me 2 dollars when at the same time i can buy the cards i want at a very low price (less than 1/4 of the booster-pack price)???...

If your aim is to build a deck without burning much cash, I suggest buying from the market as you can save alot. In my case I like buying booster packs because of the fun I get from opening them.

...but in a game like SteemMonsters don't you think that there has to be a limit on the prices???...at least a limit down ...a rule... a solid base that will guarantee that 'this is the lowest price you can sell on the market'...

No limit in the prices is actually a good thing imho. Letting the seller's and buyer's decide there own transaction limit gives more liveliness to the market place.