i vaguely remember the movie--

and as i recall the offer was made to the husband-- in which case he is selling something he doesn't "own" -- that is use of his wifes body--- and even if it was made to Demi-- she again is selling something-- Either the husband was a pimp, or she was a whore-- the transaction was exclusively about money for sex--

Marion was sacrificing something for a greater good-- (Robin and John freedom)

So i see i difference between just selling something, and making an effort for a better good-.
Schindler (of Schinders List) made money using jewish slave labor-- but what redeemed his behavior was his real effort to save the live of the jews who had been made into slaves.

He could have taken a "high" road--- and never used the slaves (and many more might have been killed) he could have taken the "low" road-- and just use the jews as slaves... He did his best to save lifes-- which meant he used slaves.. but he also worked to save the live of everyone who worked for him.

I think Marion's decision was similar.. Not perfect-- but the best she (or anyone) could do, in the circumstances.