iPhones will not send an SMS to an email address. However, here's how you fix that:
First, create your account on MsgAds.com. Log on and go to My Settings on the menu bar. Click on the "Cell Alias" button. Enter the email address your iPhone is configured for and click on the "Update" button to save your entry.
Next, using the camera on your iPhone, snap a photo of the product you'd like to post on MsgAds.com. Then go to the Camera Roll option on the lower left of the phone. Click on the picture you want to post. On the lower left, you will see a folder-like icon with an arrow, click on this icon and select the Email Photo option. Enter ads@msgads.com in the "To" field of the message. Enter the title of your post in the "Subject" field. Include a brief description of the product in the body of the message and click on the Send button on the top right corner. Your item is now posted on MsgAds.com!
NOTE: although you will receive txt messages from people interested in your post, you will not be able directly text message them back using the built in SMS application on your phone, instead, from within the received SMS text message, click on the email address and reply back using your mail client on your phone. More pending.