Not a question of legal or illegal, its illegal. period, copyrighted material. done.
Question is does a development team want to sue you for a game they made in 1994 and havent made money on since 1995. Answer No. So your not going to get in trouble for it.
If you sell things people DO make money on, movies, or xbox 360 games, you will get busted. Lets use morbid.net , was caught selling xbox games 1:1 backups. Shutdown, received 2-3 years supervised release , and a fine. finished it, started up AGAIN selling 1:1 wii, 360, ps2 backups, BUSTED AGAIN, got 2 years in prison and a $2500 fine.
On the flip side, there are mame burners out there who have been selling mame, daphne etc for over 8 years and are still up and kicking. Because noone is going to sue you for an atari or arcade set.
I see nothing wrong with selling roms, that are out of development and no longer making money in the open market, its convenient and quick for people to get games, For those who downloaded there mame set vs buying it from a burner and thinks there method of obtaining the games is OK needs there head checked. theres no difference there.