Shopping through the browser on the iPad is exactly like shopping through the browser on your computer. (Possibly more so, because there are no viruses on the iPad.) If you trust a shopping site you visit on your computer, then you should trust it on the iPad.
If you're talking about shopping through an app -- depends on whether you trust the app makers. I buy stuff all the time through the Amazon app. I haven't heard of any shopping apps available on the iTunes store purporting to be from one company and actually being from another, nor have I read anywhere about fraud.
Is there a particular app from a particular site/seller you're wondering about?