From time to time you'll need to cancel an order. Here's the most common reasons.
1. The item is no longer in stock from the source market and the repricer hadn't adjusted your quantity yet.
2. The price had increased a lot and now you will lose a substantial amount of money if you make the order.
To cancel an order go to your order history and click on the drop down in the actions column on the left. There is a small triangle to the right of the words "Print Shipping Label".
Click that triangle to get the option to cancel the eBay order.
These will be your 3 available options on the cancel screen.
Ebay won't care if you choose one of the bottom 2 options. They won't confirm this or anything. If you choose the out of stock option then it will severely hurt your transaction defect rating and that's a quick way to get suspended on eBay. And when you have several thousand listings on eBay you'll have a few of these.