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Only STEEM <-> SBD, I don't have enough TPUs to sell them freely :)

integrate steem-engine market

by just using internal sbd/steem market, people probably make losses, while using SE would also boost liquidity of your token :)

I'm not sure if that's possible :) I could either create my own exchange or users have to use steem-engine market.

can't you just do it as you did it with the internal market?
but keychain then creates a transaction for SE, which just needs to be signed :)

But who is the buyer and who is the seller?:) @tipU is not the seller for sure!

The one who wants to buy/sell and is signing the keychain transaction?

your website would only prepare the transaction

So you want me to create an exchange service :p