I wonder why such a thing does not exist yet (or does it?).
An exchange where people can place orders in an orderbook to exchange for example AdvCash USD for OKPAY USD or for Wire Transfer and so on.
For now I can only imagine a centralized way, were users sends funds first to this service, but maybe there is also a decentralized solution, I don't know (maybe Waves can do this?).
There are already plenty of shady websites out there, which will do the exchange at a fixed rate for a high fee and you send funds first. This is very expensive, very iliquid and very dangerous because of hundred of scammers.
It would be much better to have a real exchange, like bitcoin exchanges, but for these kind of e-payment currencies and with orderbook, so everyone can participate.
This way we could create on big and liquid place to exchange. Also bitcoin exchanges will profit from this. Exchanges like Exmo and others are always adjusting their deposit/withdrawal fee. Exmo has a 4% deposit fee on most e-payment deposits, because they hold too much money in these accounts and they can't exchange it savely.
So this is a very big market.
Please tell me there are some guys out there that would develop such a place.
Yes, there is a P2P exchange like you describe, with order books for getting an actionable price for any cryptoasset. Check this: https://bitcointalksearch.org/topic/m.33884473