A Trust Seal is a good idea for any web site for many reasons. The most important reason is that the web site is trusted by a 3rd party. The bad thing is, who is trusting the third party ? If the Trust Seal is provided by a well known company such as Norton, McAfee, Etrust, or Trust Guard then it's probably pretty reliable, however you are going to pay a premium price for a Trust Seal provided by these organizations.
There is a way to get a Free Trust Seal that is backed by Google, Norton, McAfee & Sucuri that is provided by a service called A Safe Site however you are going to see some ads on the results page that is displayed after clicking on your trust seal or you can opt to get 10% off Trust-Guard.