Even if you do re-use a box and packing materials (not that you can re-use tape) it's totally reasonable to apply a small packing charge. I sometimes spend 15-20 minutes packing an item (although that's an exception). More often it takes 5 minutes (to do it properly, not just sling it in a box). If I was paying myself minimum wage to pack an item, that'd be just over £1. I generally add 20-50p packing charge to my postage cost, so I'm already paying myself less than half minimum wage to pack an item.
Ebay will say - just add it to the item cost, but for some low value items that just means they won't sell. Ebay will become a vast warehouse sellers don't want to sell on, and buyers think is overpriced. Also I think there's value in being able to communicate to buyers that £X is for the item, £Y is for the postage and £Z is for the packing. When I buy on ebay that feels authentic to me, I know things cost money to post, and I know if I want it packed well it's reasonable to pay a little for that too.
If every cost is contained within the price of the item, buyers will just go "how on Earth is that DVD worth £6?" without realising it's actually worth £3.50 but postage is £2 and packing is £0.50.
A bit like how ebay users today have largely 'forgotten' it's conscientious to provide positive feedback if you have a good transaction, and that sellers are not all businesses they can be rude to. I'm just a bloke in a house mate, stop treating me like you would a shop assistant, and stop treating shop assistants like dirt while you're at it.
Oh, I pine for the glory days of the early 2000s.
