I like Telegram bots and it is cool idea, but mains issues are private keys and escrow. How are you planning to handle that?
Private keys on the server, same as any other hosted wallet or exchange (using BitGo),
and bot will provide escrow by holding bitcoin deposit until seller confirms receiving deposit. With manual resolution in case of disputes.
I'm open to any suggestions, it's all in very early stage of development, currently running on testnets.