I always ask "Are you working with a realtor right now?" before showing them ANY property.

I always get an Exclusive Buyer Brokerage Agreement signed by the buyer at the time they write an offer. This stops any other agent walking in to take MY commission - and if they DO, the buyer is on the hook to pay MY commission, too.

I once had a buyer do a dirty on me. The buyer wanted to see a property. He said he was working with his realtor-friend but she had been unable to get him a property in six months of looking. I explained to him that if I showed him THIS house, I would be the realtor if he wants to buy THIS one, but that he can go back to his friend for other properties. Ok, he says. He likes it... but it's not quite right. He asks for more properties. I ask him about his other realtor. He says she's a friend and he'll make good on it for her. He says she never found him what he wanted but he likes the properties I have shown him. I showed him about a dozen houses in 2 days and he finds his ideal house and wants to make an offer. I draw up a contract. He says he'll sign it in the morning. In the morning, he calls me to ask if his realtor-friend can take over and he "pay me for my time". Nope. I explain to him in detail about Florida's procurring cause and how my $6500 commission earned in 2 short days is worth more to me than anything he could pay me off with.. unless he wants to pay two commissions (one for me and one for her). He says he understands it. He decides he wants to go back to his realtor-friend to look at other houses and will avoid this dream house. Many months later I have this idea to look up the property. It's sold. The selling agent is the name of his realtor-friend. I call the listing agent to find out the buyer. (The title hadn't made it to public records yet.) It was my guy. So I tell my broker. We do a procuring cause case at our local realtor board. I win.