These type of services are not your friend, I can't tell you how many times I get anonymous inquiries on my higher value domains. When I ask them to fill in my long contact form I meet resistance and I end up ignoring the inquiry.
I found most of my legit buyers that truly wanted the domain would fill out the form and we would negotiate above board.
On the anonymous inquiries I often ask them why I would want to negotiate not knowing who I was talking too.
One of my domains (a 4 letter .CA) is constantly getting inquiries from a broker and end user and the broker fills in all kinds of bogus information and eventually bits and pieces about themselves. I tell him clearly I do not sell to brokers unless they disclose information. Then he makes things up and I make him aware of the disclosure at the end of my long form.
I tell him acquiring a domain under false pretenses can have legal ramifications and then it goes silent for a while.
My short form to get inquiries: mapledots.ca/offer/?domain=red.ca
My long form needed to actually acquire the domain: mapledots.ca/inquiry/?domain=red.ca
Notice the note at the bottom of the long form.