In short, if you qualify and buy the endorsement (under the terms specified), I give you the bid for free with no kickbacks.
You want a $50 endorsement on your loan. You contact me, we talk, and you send me $75 through PayPal. I bid $50 of that money on your loan with an endorsement and keep $25 for my time.
But you're not bidding with your own money; you're bidding with the borrower's money. If the borrower paid as agreed, you'd get "free" interest payments on top of your $25 fee. If the borrower defaulted on the loan, you wouldn't lose a dime. So you actually have no financial incentive to choose good borrowers.
It doesn't matter that you "would have" bid on the loan with your own money, if in fact you are using the borrower's money instead.