For example, AppAnnie, BuySellAds, Blurgroup, etc. etc. I'm new in the field and would like to start off with hearing some genuine opinions about the current players
Please explain further what you mean by marketing buying platform because there are many options. If I think I understand correctly I am most acquainted with Solo Ads.
Freelancer.com and Odesk.com are similar to Blurgroup but targeted towards smaller projects. If you're into media buying you might also consider doing some PR. You can buy your way into forbes.com etc. Articles on major digital publications usually cost around $1000 USD. Otherwise if you have some really exciting news just do a press release.