He literally said why: he didn't want to to get diverted from revenge, and because he fell in love with her, he was tempted to choose HER over revenge (ie, live a peaceful life with her and not try to take over the kingdom).
He chose to sell her to be rid of that temptation.
But man, why didn't he just tell her to leave or been mean to make her leave. Instead , he sells her to someone who admits they are going to use them viciously/sadistically. That makes no fucking sense. (Although I guess he'd say he needed to money for revenge plan.)
The JACKASS got a castle in exchange for her not money the bITCH CHOSE A USELESS FUCKING CASTLE OVER THE GIRL HE LIKED. THAT BITCH SHOULD BURN IN THE FUCKING DEAPEST, DARKEST, SOUL TEARING, MIND CRUSHING, PIT THAT HELL HAS FOR ALL ETERNITY WHILE BEING TORN APART CELL BY FUCKING CELL!!!!!!!!!!!!!!!!!!!!!!!
Anyway hope you have a good day (=・ω・=)
That's it. A castle. Forgot that. But I knew he got something valuable for her, and he knew the person was gonna abuse Bella. That's the thing that really pisses me off: he knew she would be beaten. Bella should have run away, though. I know she stayed to wait for him to show up (he didn't), but fuck that. Just leave. Go live a life without that useless shithead.

I may be stupid but can someone tell me what's the hidden reason why he sell Bella away ヽ(`Д´)ノ