I will reply on what I know ...

They mine other coins and then they buy BlackCoins with the other mined coins right?
Right

How do they buy BlackCoins? Do they put Buy Orders or they go directly to the sell orders?
They do not share this information. Logical, because the market should not know what you plan to do.

Whats the buy volume of all pools like this, lets say peer week?
Well, now I do not think that it is too much since the biggest multipool was DDoSed and started again to attract miners since yesterday. If I remember correctly, in its previous peak, multipool had something like 50 BTC per day.

I'm thinking.. If they go always to the sell orders and the buy volume is high, then we will have a BlackCoin infinite increase in value... ?
Good point, but it depends very much on the price and on the hashing power. I mean, If the price goes 10 times up in 1 week, it is difficult to attract 10 times more hashing power in 1 week only.

Any place where I can verify the volume of these pools, and how many pools exist?
I know for sure
http://blackcoinpool.com
http://www.cryptoalts.com/
and I think that https://hashco.ws/ offers the possibility to get paid in BC.

That's all I know and I can remember ...