As the title suggests, I think it would be a good idea to allow the buyer and seller to communicate within the app. Probably only temporarily until the trade is complete. It could be enabled by default or it could be required for both parties to enter the chat first. The main reason is, I think it would help resolve many disputes without the need to way for an arbitrator to get involved.
There was talks about this multiple times, but I guess devs just figured it would cause more harm, due to social engineering attacks, than good.