Everybody hates to be scammed. If you are looking for expert help in selling and buying properties, you will surely runaway from the fake ones. You don't want them to eat up your money without getting any benefit from the transaction. The question now is: how will you know you're hiring a trustworthy agent?
First - the person you hire is loyal
Yes, it is true that you don't know his whereabouts most of the time. But you can always figure out his sincerity with the way he treats you. Loyalty in this sense doesn't mean you're the only client he will work for. Unless you are selling a lot of properties, he can't do that. He is loyal if he remains true to his commitment with you even if he has other tasks and clients to attend to.
Second - he values customer service over profit
You pay for his commission right after the deal is closed, right? Whilst things are still under negotiation, he must see to it that you are satisfied with his services. He should have no reasons to turn his back from you if you say you're going to pay him only this percentage for the deal.
Third - he gives you a realistic idea on the deal
Estate agents know the real estate arena more than you do. He must be able to give you not an opinionated idea on the deal but a realistic one. Let's say you are selling a home then the agent tells this is the price you can set. You can always trust what he says. But the better way to do it is to make your own research. You can come across online valuation websites that can give you an answer to your questions.