MyPayingAds is an advertising site that offers various advertising tools as well as earning opportunity to members through revenue sharing.
It is free to join. 
YES it is legal. In order for any program to be legal, it is required to supply a service and/or product to you, the member! 
All members that purchase ad packs will get Business listing credits (20 credits for $1 value of ad pack purchase). 
More details are available in the "Details" page.

https://www.mypayingads.com/index.php?ref=99226