The negative feedback left by Yahoo is more than enough for such a scenario. Don't sweat yourself off over a trust flag mate. Anyone sensible would be able to notice that the accounts are alts and the person behind them tried to cheat on a bounty.

If we go one creating flags for issues like this then trust flags will become meaningless. I have seen so many accounts that have been busted for cheating on bounties... probably thousands of them.
Are you going to create flags for all of them?