You click on 'More Refinements' on the left hand side, then click 'Item location', then 'Within ... distance of [postcode]'. There isn't any option for 30km, but you can select 25km.
However, if the seller just has their location set to a main city (i.e. does not show which suburb), it is always good to check with them, as I set my distance to 10km from my postcode, and it turned out the seller had moved 50km away and not updated their location.
