Been both sides. I bought a wheelset, including postage for a bargain price. Seems the seller didn't get what he wanted for it because he then said he'd miscalculated the postage cost and was pulling out. So I offered to go halves on the postage with him (so the wheels worth ~£150 that I'd got for £100-odd posted were still only going to cost me £110) Then he said that he was going abroad and couldn't post them, so he was pulling out. So I told him that was fine, I'd wait till he got back. By this time I was determined to get them off him, which in the end I did.
I also sold some stuff recently. Priced up on ebay properly as a small parcel first class and charged accordingly on the listing. When i got to the PO the counter bastard said he wasn't allowed to squeeze the parcel through the posting slot, it had to pass through easily (it was clothing so compressible). Which then meant that the parcel cost me £8 to send as a medium parcel 2nd class instead of the £6.85 I'd quoted.
The recipient then messaged me complaining that I'd quoted first class and sent second and he wanted me to find out the difference and refund it (£0.90). I politely pointed out that if he wanted me to requote I'd be doing it for the medium parcel it had cost, charging him the full £8. Or alternatively, as he already had the goods in his hand, and was happy with them, I'd suck up the extra postage cost and say no more about it.