@Carlos Jonathan Fuentes I have a system for this and it is something to pay attention to, but as others have said the agent-investor relationship is one built on trust so sometimes you will lose and just remember what went wrong.
If I am working with investors looking to buy and live in the property or hold, I don't ask for a buyer agency agreement, but I use something like that by getting an agreement on the contract that they will be paying a commission. I write the contract up for the off-market seller and include the commission to buyer on there. But, see next bit about working off-market sellers for a commission.
If I am working with investors looking to flip and I source the property for them, I charge the off-market seller a small commission for bringing them the deal and I run the deal as dual agency to keep the transaction friendly and on track. I do not charge the flipper a free because we have a handshake agreement that they will list the flipped property with me.
In all aspects, I disclose the commission to both sides and make it clear on the second point that I will end up earning two commissions, but I will show them how I earn it by bringing the deal together and then marketing the high-end flipped property to the max.
I hope this helps. PM me for more info.