Variation of this idea have been tried before. A couple of issues that will need to be overcome.
The use of Paypal, credit cards and anything to do with a bank (other then cash deposits) will always be non-starter. Conventional payment methods such as those are reversible. Cryptocurrency is not reversible.
When BTC started many people got burned by scammers that would tender payment in Paypal and such, then once they received the coins, turn around an reverse the payment. The coin seller had little recourse in that situation.
The part about rating transactions (like a Goldcoin WOT) built into the client is a nice idea, but some sort of moderation needs to be built into it. Blackmail could become a serious problem otherwise."Unless you give me xxxx GLD I will give you a low rating and label you a scammer". While transactions are transparent, who holds the keys to an account is not.
Perhaps a combination of a moderated trade site the clients could hook into would be the answer. Like an escrow/trading site that one could browse from the client.