Or another type of bot. A want to sell POCKET for Steem and sends them too a bot with a command in memo:
pocketsend;1000@thebotname,10 STEEM
The bot waits for a transfer of the amount specified in the memo. B make a Steem transfer to the bot with @A as memo, the bot transfers the POCKET tokens to B and STEEM to A, takes a fee if needed. If no transfer has been done after X days, then POCKET tokens are refunded.

A can advertise the offer. The bot can reply to commands and can also have a presence on Discord for listing all available offers.