Right now you can use private messages on Steemit chat or Discord for setting up those types of transactions. https://steemit.chat

If you join the Minnow Support Project on Discord they have a chat room called #the-swap where you can buy, sell, trade, exchange... whatever.

Below is the invite link - go to #start-here-rules chat room first and it will tell you how to register to get access to all the chat rooms and to get votes from their bots.
https://discord.gg/GNDDfw