I did some research and found out some info on this!

Basically, these people buy a virtual credit card for about $5 or $4 or so from some third party and then create an account with Adwords, YSM or Adcenter.

Sometimes these will provide bonus or they may use a voucher that will increase the credit.

Then they sell it at forums for a small profit. I have heard good reviews about these and many people have claimed good results.

However, one has to take certain precautions like hiding IP, using throwaway domains, etc as it will be difficult to create multiple accounts without detection.

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