Display selected option when editing a form (Rails 4)

<%= f.select(:condition, options_for_select([["Brand New", "Brand New"], ["Pre-owned", "Pre-owned"]], :selected => f.object.condition), :include_blank => true) %>

Check the options_for_select documentation, and you will discover that the last parameter is the selected option.

options_for_select(container, selected = nil)

In your case

<%= f.select(:condition, options_for_select([["Brand New", "Brand New"], ["Pre-owned", "Pre-owned"]], params[:condition]), :include_blank => true) %>

assuming params[:condition] contains the currently selected value, and the value matches the corresponding value in the select tag.

In other words, for "Pre-owned" to be selected, params[:condition] must contain "Pre-owned".