ok folks, I did it.
Here's how:
First, I needed to add Poltergeist to Capybara and specify it in my test, like so
it "can decide price", js: true do
Second, I had pre-existing javascript errors that needed to be fixed before Poltergeist would even run. Third, this code wasn't connected,
<%= f.label :want_sell?, 'Want to sell your item?' %></br>
<%= f.check_box :want_sell, :id => 'want_sell' %>
it should have been like this
<%= f.label :want_sell?, 'Want to sell your item?' %></br>
<%= f.check_box :want_sell?, :id => 'want_sell' %>
so my test spec could click the label as a reference to the checkbox.
now i can simply write in my tests...
check 'Want to sell your item?'
Total missions bruv.