For cash trades, I don't think there is a point to force traders to keep the conversations on the platform. We offer the option if you want to chat on the platform but if traders want to talk outside, it's their call.
It depends if you are going to be offering escrow or trying to verify trusted feedback or not. If you are going to do either of these things then you absolutely want all communication to stay on your platform so it can be reviewed in the case of a dispute.
For this problem, we could add a simple interface that allows both traders to optionally lock the price on the platform according to the platform price to avoid argumentation and even provide the bitcoin amount + btc transaction fee they need to transfer at the agreed rate for the agreed cash amount.
That's a good idea, but there is still the issue of people sending with too low a fee despite the fee that you suggest, or sending with a high fee and still having to wait over an hour for a confirmation since blocks are not mined regularly. No one wants to make an anonymous cash trade and hang around with the other person for an hour. The only way to solve this is for the buyer to accept a zero confirmation transaction and take the risk of losing coins to a double spend, or to use an escrow.