I got in a real situation with a buyer, i sold this lovely set of mills and boon books, on the ebay ad for them I put my little protest, words to the effect "ebay say they are just a venue, and yet this set of 13 books offered for starting bid 99 cents, I cannot charge the proper actual postage costs for because ebay have a postage cap" in effect if the books sold for the starting bid of 99 cents (which they did lol) i would lose money because express 3 kg envelopes are $11.30 not $8.30, so I put on the ad that the winning buyer would have to pay $11.30 not $8.30 (i.e. $3.00 more) if they won despite the listing saying $8.30 for postage costs.
Anyway they sold for 99 cents, the buyer paid straight away the 99 cents plus the $8.30 on the ad and expected to receive them for that, a very nice buyer btw not difficult, paid fast etc.
So i email them say did you read the ad etc, and no reply, in the end i refunded the paypal and sent a new invoice which the buyer paid, with the proper postage but a huge drama for both buyer and seller to make a transaction work.
ebay cannot argue it is "just a venue" it is clear it is not, a venue does not tell a seller what to charge for postage NOR force a seller to discount the actual cost of postage.