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One simple way is that each time a buyback occurs, have a copy of the transaction threaded out. Set up an account for this so there is a list of the threads there with each transaction. Then they can be passed around.

Also place a counter on the site, perhaps on the dashboard or leaderboard page (if you dont want to do front page) of the total buybacks. Of course, the front page might be better because then people can see it growing before their eyes.

Call it Leo purchased off the open market.

I have to agree with Task here.
Showing the buybacks and them occurring on a regular basis will give some faith in the process.

It might only be of interest to the investor types tho