1 - yes (depend on price of course ; I will say may 500$)
2 - I don't personally do that, but I don't see any thing wrong with that. If I would do, I would put a reserve price.
3 - Detailed size of the bike / first hand or not / original bill or not ; purchase history (previous owner, .../ equipment / detailed pictured of any defaults.
4- yes for sure. There is so many bike stolen, that buying second hand needs some check before purchase. (question of self prootection, and morality too. I of course don't want to purchase a stolen bike, even for a good price).
When I purchase a bike (generally locally and after a real discussion with a real guy, I always ask for a picture of his identity card / driving licence. Of course I provide mine. If the guy is not ready for that, I go my way.
We do that for a car, why not for a bike ?
5 - Payment. the platform should collect the payment of the buyer until the object is send, checked and approved. Then pay the seller.
No direct transaction , even with paypal or similar. (even if paypal is a great way of paying)
It already exists, but not on every platforms.
I suppose you understood that I prefer by far dealing with a real person in front of me, and looking at the bike by myself.
I don't use ebay myself, but I use other selling platforms in france. (troc velo , le bon coin, ....)