Stupidity: So he can rescue his wife from slavery, Django comes up with a plan to buy Candie's most expensive fighter and then get him to throw her in for free. Why doesn't Django just offer to buy her directly? Surely there was some amount that Candie would agree to. Even racists like money.
Suggested correction: You missed the point of the plan. They knew if they went in asking to buy Broomhilda directly, Candie would set the price too high. They feigned interested in his best fighter and would get him to throw in Django's wife at a nominal price. They would then just pay the nominal price for Broomhilda and back out of buying the fighter. It's only when Candie is told Django and Broomhilda know each other did he raise the price for her.
Yes, Candie, not Candle. Stupid typo on my part. I disagree with the correction though in the sense of why would Candie raise the price before knowing that Broomhilda was Django's wife? You yourself said in your correction that he only did so when he found this out. They could still have offered to buy her initially.
Because if they didn't feign interest in buying a fighter, Candie wouldn't have even invited them to his place. So the plan was to get him to throw her in for free, rather than risk him setting the price too high (or not even negotiating at all). Candie even figured out what their plan was.