I paid about £700 for a used set of UK's front and back. Condition of them was worse than I was hoping for and I needed new discs and pads all round as well as a lot of elbow grease to clean them up. If I'd known, I'd have not gone higher than £500 for the set. It cost me about as much in the end to fit used as it would have to fit new.
If yours are in decent condition with regards to discs and pads, the parts to rebuild aren't that much. It cost me about £260 for parts to rebuild both fronts a bit ago from Chris Wilson, worth checking though as prices may have moved. You could possibly do it cheaper if your pistons are in good condition and not rusted to buggery. In terms of how easy or difficult, technically not hard just time consuming. I'd probably leave it as an exercise to whoever buys them, just take good photos so they can see what they're getting into when you sell.