Why did I get the error "Sorry, your last action could not be completed" with my Yahoo store?

This error can occur because you're using URL masking or framing, or have a Yahoo Store with domain forwarding enabled.

Find the following line in the HTML code of your PayPal Standard button:

<form action="https://www.paypal.com/cgi-bin/webscr" method="post">

Add target="paypal" to that line, so that it matches the next line:
<form action="https://www.paypal.com/cgi-bin/webscr" method="post" target="paypal">

Yahoo! users can also downgrade to standard forwarding, or turn off domain forwarding entirely.

For more details, see instructions from Yahoo! on setting up domain forwarding.

See also:
How do I contact Yahoo support