I want to give the Aperture back

From the Mac App Store FAQ

What is the Mac App Store refund policy?

If you experience a technical issue with an app, first contact the developer of the app for assistance. Their support contact information can be found on the app's page in the Mac App Store. If that does not resolve the issue, contactMac App Store Customer Support.

Of course Aperture works for the majority of users so if you are having problems it is probably something on your system or in your setup that is at fault.

If you want to try and get it working describe your situation, system OS version, etc. in detail and perhaps you can be helped.

If this was not an honest request for help them contact Apple to try and get a refund.

regards

In addition to what Frank Caggiano said:

If you sign into the AppStore, there is a link to the "Terms and Conditions" at the bottom of the page (in very small print).

http://www.apple.com/legal/internet-services/itunes/uk/terms.html

You have the right to withdraw from your transaction without charge and without giving any reason until delivery of the product has started. You do not have the right to withdraw from a transaction once delivery of the product has started at which point your transaction is final. The Stores’ services commence immediately when you begin to download a product from the relevant Store and you will not have a right to cancel your contract once the services commence unless you have received an unacceptably poor download. If this happens, you must tell us as soon as possible using the “Report a Problem” link in your invoice, and if the poor download is caused by iTunes and cannot be resolved, a refund may be provided.

Note: Once the program has been delivered, your only option to cancel the purchase would be a poor download, not if you do not like the program.