I push domains all the time with no payment and I almost always get domains from namepros members without payment. However, I feel bad if I don't get the payment to them in 10 minutes. I once had partial payment held by paypal and I communicated with the seller at all point informing of the issue and what was happening.
In the years I have done business I have almost always worked on a handshake.
That said, I mostly pay paypal or bank transfer and in both cases the transfer of funds are clearly marked FULL payment for domain.com. If I don't get the domain I get the money back from paypal and I can get it back from my bank.
Documentation is the key if you are going to work on handshake. I have been very fortunate because I use a bill of sale and have a very in depth contact form. I traditionally research my clients well before the sale actually happens. I find too many domainers don't have enough info when selling a domain. The standard name, telephone and email really does not cut it.
Have a look at my form www.mapledots.com/contact and word something similar. With that type of information you can easily track the buyer or get your funds back.