If you buying a text link on some domain or buying a domain (or service) > << example.com >> Ask the seller to give you an email address of that domain name, like You have to send an email to that address. In the email, mention that this is the response to your ad on DP (with the ad URL). Mention all the terms and conditions in the email. Ask the seller to reply back as "I Agree" to your email for whatever the deal may be. Now, before you send the email, there is a system where you can track if the email was read or not. Its worth the money you spend on the service but you will have strong evidence just in case you get scammed. (You have a free trial too) Its called "ReadNotify.com" As soon as the person opens your email, you will get an email with a lot of details like his IP address, his location, his ISP details and a lot more details..like "digital signature" too which can be an evidence to catch hold of him and give him some good Bashes on his face! With these details, you can even lodge a complain in your local cyber crime department. Ive learned this hard way after losing a lot of money! Hope this helps!
A simple way to see if a email was read place a IMAGE from your host. If the image was downloaded then you know that the image was read. You can even put in vars in the image to verify.
remote images normally blocked by default in several mail programs / mail providers. so it wont be accurate