Its a toughie tbh - I don't actually agree with people charging for their time and bus fares, the hassle of lugging a buggy around etc to post things - I think for the vast majority of people when they decide to ebay something that is something that they should take into consideration....if its SUCH a hassle for them to post the item, then maybe they should consider whether selling via ebay is for them rather than NCT sales or car boot sales etc which may be less "hassle" for them. I also doubt for the vast majority of them even if they have to take a bus to the post office, I doubt posting my item is the only bit of business they are doing at the shops that day, and frankly I'm against me personally subsidising their shopping trips.
There are also a LOT of items that are sold on ebay that don't require a trip to the post office - dvds, cds, and a large amopunt of childrens clothes will fit in a normal post box - print out the postage online at the post office website (which will also give you an accurate price that the item will cost prior to listing), and no trip to the post office is required - just pop it in the postbox when you pass.
On the odd occassion that I sell something on ebay (and the reason I don;t sell more is because of the huge pfaff I find it to take the photos and write the listings), I ALWAYS charge the amount it will cost me to post (gleaned in advance from the post office website). I generally use recycled packaging, or if its a dvd or a cd, a normal envelope picked up in packs of 25 or 50 at the pound shop and the supermarket (and frankly I don;t have space in my life to start charging people 4p or 2p to cover the envelope). I always say in my listings this is the case and I hope that when people are comparing my listings with the listing of other sellers that they will bid on mine rather than theirs because they know I'm not trying to fleece them for extra money, or for money that I'm avoiding paying fees on (ebay fees aren't charged on p&p).
On the whole, when I buy from ebay, I factor the amount of the P&p into whether I bid or not - esp on children's clothes where sometimes you can be paying a small amount for an item, but the p&p can really bump it up. A top might be a bargain at £2, but when it actually costs £4.50 with £2.50 p&p then its not a bargain.