eCommerce for event registration?

If you only have one even at a time to sell it might be overkill to load a whole ecommerce package. PayPal and Google Checkout both generate html for buttons that you can paste into a page or post(using HTML mode of the editor!)When you get the sale your accounts with either option will collect the buyers info for you.
If you need more functionality than a button I use wp-e-commerce on loads of sites and a few of them for making an event a product and collecting fees.