I was selling some books and they were big hard back books. I weighed them and did the royal mail price finder but then once I'd put all the details into ebay they said I could only charge up to XX amount in postage but it was less than the Post office price finder said it would be, meaning I'd have to take a loss on the postage. I didn't bother listing them once I realised because it wasn't worth it, once you've paid the listing fee, the paypal fee and then taken a loss on the postage you're left with nothing.
If you put £0.00p postage costs, there will be someone who will pay before you get a chance to invoice and then you have to chase them for postage costs, or faff about refunding and then re-invoicing.
For books you can add 'freight' as an option and then postage costs detailed in the listing. I've done this a few times when selling Hardbacks and Series. It means you don't pay additional eBay fees on it, as postage costs are not included.