in theory though, the modified firmware is illegal. which means if you sell it you are selling a 360 that is illegal. The odds of you getting in trouble is small but its there and I guarantee you law enforcement watches craigslist/ebay for modded xboxes because getting an illegal xbox would give them evidence to get a search warrant to search your premises as it could possibly lead to more illegal things such as games, movies, music, etc. However the chances of that are tiny as little ol you isnt who they are after but anytime you do something illegal, you run the risk of getting caught.
think about reading the other threads which have talked about this.
Modifying the firmware to circumvent copy protection is ILLEGAL. Have you heard of the DMCA?
Selling a 360 which has the hacked firmware (hence selling an xbox which can circumvent copy protection) = ILLEGAL. The only way I can see you being capable of selling the 360 is by advertising it WITHOUT mentioning its ability to play backups and without mentioning the fact that it has ixtreme on it.
It's still illegal, but odds of anyone finding out you have hacked firmware on it if you never mention it = < 1%
