When you have nonactive domains, is possible that you consider the option of sale or park it, is it normal. When a domain come for sale, the notice will be posted in the web which you used to sale it or in third party affiliated webs. This is for maximize the probability of sailing the domain (this helps you and also report a benefit to the web where you publish the notice of sale). At here, all good, the problem comes when somebody try to cheat yourself.
The fraud is
Someone contacts with you via email (out of the jurisdiction of the website where you decided to publish the notice of sale) the buyer shows interest in the domain, the first step is asking you. How much it cost?
In the next step the possible buyer accept your prize, but with one previous condition, the domain must be certified. What does it mean? Easy, there are many legal companies that offers domain evaluation in order to guide the seller and the buyer. In our case is slightly different from the legal, the buyer insists in one of those. It justify with links to false forums or blogs.
The problem comes when you finally accept to bring him a certify, it have a cost. The prize start from 14 to 200 dollars. When at the end you acquire it and resend to the possible buyer he will disappear, producing a 100% benefit to the illegal certify business or a direct benefit to the fraudulent buyer.