Choosing competitors for your listings to reprice against, can be done by auto-select (based on selecting competitor listings based on the eBay search list) or via the the eBay Item Number to target specific competing listings.
The simplest way to choose competitors is by automatically selecting. The auto select features searches eBay with your product title and then automatically selects any competitor who comes up within either 30% below or above your current selling price. *The reason to only catch 30% above and below is to exclude products in eBay search that may not be like for like. For example searching for an Apple iPhone my bring up cases and mobile phones which have vastly different pricing.
Auto Select Method
To use 'Auto Select' for all listings simply click on click on either 'Remove competitors for all listings' or 'Select competitors for all listings'.
Target a lesser number of sellers in bulk using Auto-Select you can yes the check boxes in the left column too. *Remember the search is intelligent, if you search for 'chargers' for example it will show all products with the words chargers in them regardless of where the word is positioned. Then click on the 'auto select' button.