CPM vs CPC , who decides as a publisher ?
 

some sites are selling traffic for CPM and some for CPC so ,

who decides how you should sell your traffic ?

is it the buyer or advertising broker/network or seller.

isn't the CPM always expensive for advertiser.

It depends most of the overall income after all because if the advertiser does not gain much out of his campaign, meaning that he does not gain $xx per signup and gains just $x he will most probably go for CPC which would insure him filtered traffic as only those interested would click.
However if his targets are high and he has a pretty well made campaign with "expensive" outcome he'd go for CPM as it's much more attractive to the publisher although it may produce a lot of garbage traffic.

thanks for your input.

because as a advertiser when i go to buy traffic those fuckers offers me on CPM and while as a publisher when i go to sell my traffic they offer me CPC.

looks like big traffic sites own these traffic networks.

Most ad networks are CPM nowadays, it has more benefits than CPC, the main one being that advertisers will update their creatives to keep CTR high. CPC encourages laziness in my opinion, you pay every time your ad is clicked so why bother how good your ad is? It encourages competition and lots of testing different kinds of ads. It keeps the sites fresh as well, everyone wins.