I sell and buy. When I sell two items with the same postage, I just charge one set of p&p.
Also, if I sell something with £2.85 small parcel postage and manage to send it as a large letter instead then I refund the difference and let the buyer know. I thought this was basic good practice (and honesty!).
I have had two sellers who have agreed to combine postage in advance but then only discounted it. So, three small items that were all being (probably cheekily) listed with £2.90 postage would still fit into one small parcel. She wants to charge me £5.00, saying it's fair and take it or leave it.
I know we have a choice not to buy but cancelling a sale is a black mark and I find it odd that people are so blatant about postage rip-offs when they are scored specifically on it. I know it's currently at the seller's discretion but why doesn't eBay set a definition for combined postage?