It's all about easy and convenient trade, not about making profit out of some player - anybody can check the current price.
But just imagine it's in the middle of a dungeon run with like 1 hour till the end and the item could be used immediately. The trading post is far from a perfect solution, even though it's good enough for 99% trading needs.
There can be many possible scenarios when a direct player to player trading would be desirable and when without it many things can go wrong, examples:
- Any item sale could be done through the Trading Post but every item bought has to be manually picked up at the right spot - the trip can take up to even 5 minutes (large city loading times) +extra maptravel fees.
- A particular item may be rare/unpopular enough that there's a large spread between buy and sell prices and the only ones available are significantly overpriced or not available at all (happened even to low level items on Sunday BWE3, likely because people rather threw it to the Mystic Forge than wait for sale.
- I am in a dungeon PuG, a party member asks if "anyone has a decent lvl 50 Scepter? because mine is poop and I could use the Scepter skills for this boss" and I happen to have a nice one but it's worth more than a "free item" so I could sell it to him for a fair market price.... but NO, Anet want's him to buy it on BS Trading Post and make the party wait 5 minutes till he comes back!
- Barter. Infinite possible scenarios when 2 players have items the other wants and because they have already found each other they don't want to do complicated double-sells with double fees and extra travelling but freaking simply swap the items!
- Last but not least - it's a scammers' paradise. "WTS [item], just 2g99s, you pay first" ... extra spam, extra work for support teams, and even that casual trade with a PuG member you've played for 2 hours with you can be never sure about...
Such a simple feature every other game has...
Personally, I prefer the 99% working solution. The last 1% would be just icing on the cake. But, to each his own.
As for your scenarios:
- The extra travel fees are minimal at best. In fact, it's much lower with every BWE that I've played. The largest sum was, what, 60 copper, and that's because I was traveling maybe three zones away from my destination? Granted, we haven't actually seen a larger portion of the game, but I'd expect more cities to pop up at later zones than you needing to constantly travel back to any of the starter cities and/or Lion's Arch. And, as far as I remember, there are Black Lion Trading Posts in the starter zones outside the cities, so you can eliminate the "long" loading time issue. If you desperately need the item for any reason, then you should be prepared to cover the fee. Plus, if you made any sales at all other than that single item, then your fee is a non-issue.
- I don't see how that's a problem, unless you mean you want to trade the item directly to another person at a price different from what's offered at the Trading Post. That being said, I do support the option for Cash on Delivery.
- This is ... how do I put it, strangely off? Items drop to your actual level frequent enough that you don't constantly need to look for upgrades than necessary. There is rarely "poop" item of any kind, unless it's sellable junk. I had three staves (and a smattering of other items) dropping for me around my level when I already have a decent one for myself. If the PUG player hasn't been keeping items for himself that he should and realizing that he has to use one later, then I don't see how it's a fault of the trading system. For one, having a dungeon run fail because of not upgrading ONE weapon properly is hardly realistic. If he even had a Level 50 white Scepter, that is good enough - blue rarity and above just makes him slightly better. The scenario you're describing is more that he's using a starter Scepter in a Level 50 dungeon, of which then it is the fault of the player instead. Two, if you can't wait 5 minutes out of how many hours you've spent in the dungeon, then I don't know what to say. Remind me of a time when there's no Warlock in your team to put up a Summoning Portal for a party member who forgot to bring extra mandatory consumables, deep into a raid.
- I can see this being a problem, yes, if you have an issue of trust. Like you said, both of you have found each other, and that means both of you have said items ready to be traded. The only reason why you wouldn't proceed with it is because you don't trust each other after all (paradoxical much)! You either have to be worried about getting said item at all or getting a cut of the transaction - I don't see how hard is it to see which one is better.
- I'm pretty sure one of the reasons quoted by many on the non-implementation of a direct player-to-player trading is to prevent scams like that from happening. Any player with any inkling of how to trade at all would know ignore these "spammers" (I mean, even with the blatantly obvious "you pay first" tag ...). Granted, there would be players eventually falling to the scheme, but I think the system put in place, imperfect as it is, has the aim to protect buyers first and foremost. Personally, I'll be ignoring people doing "WTS"es, and that eliminates a lot of headaches (for me, at least) that you purport will happen.