You need to build some trust with them and provide value throughout the process. If you introduce two parties that would have had a hard time finding each other, and help with questions and suggestions throughout the process, then they'll be happy to pay you an want you to do that again.

If you just give them each other's names and email address / phone number, then want a big check a few months later, then they can (and should) skip you.

I would model your contract after the way houses are sold in your area. Real estate agents face the same problem. Or you could look at how headhunters do it.

How can you avoid that they skip you? Get a contract in place.