There are fairly new restrictions in place on certain categories on eBay, and books is one of them. I sell books sometimes and the new rules mean that (as with Amazon) I can only charge £2.75 maximum for p&p irrespective of the size and weight of the books - the postage alone can be £4. under these circumstances I always show the weight of the book in my listings and mention that the price has been inflated to allow for the real postage prices (most of my paperbacks are on for £2 starting price but hardbacks will be more, just so I can recoup the actualy cost of postage. Otherwise I'd be paying my sellers to take my books away!
As molly says, the other point to remember is that we are rated as sellers by buyers on our postage and packing charges, which is why - if say I have charge £6 for p&p and the actual postage was only £3, because I miscalculated - then I will refund part/all of the difference back to the buyer, to keep everyone happy. I know not everyone does this - I bought 2 things from the same buyer and they did not offer an option to combine postage for 2 things in the one parcel, so they got a rating downgrade for that.
I agree that some sellers do seek to profit on P&P but eBay does remind buyers that we are allowed to charge for postage, packing, getting the parcel to the PO if petrol is needed - etc., but the overheads need to be reasonable.