Thank you guys, thanks Dan. You made it clear to me. See no reason to sell unlicensed GBA rom's. I have no more question for this thread.

Well, really, if you look into the Sega v. Accolade case, you'll find some very interesting information. Basically, although originally an injunction was served over Accolade for working out how to use the Sega check, and using it. However, this was later cancelled, and has been a key - and heavily quoted - case, for proving why we (GBA-devers) are not breaking the law.
Nintendo won't really like you selling the games, and will probably threaten to sue unless you stop. However, if you sell them to not make a profit (pretty much impossible if you are just supplying the roms, not on single write cartridges) they might turn a blind eye. You could, as has been said, supply the games un'patched' and simply tell them what to use (such as the rather excellent GBARM available on gbadev.org) may work.

Hmm. Interesting. But it's risky... In such situation you always have a chance to get sued :(

Yep, it will always be risky, until you get licensed or get a licensed publisher. I just make GBA games for fun, like most programming I do, and also to have the 'buzz' of working something else that's been bugging (ahahah) you in your game. Make some quality, free titles, or demos - for example, from the gbadev.org 2003 compo, the 'Sushi the Cat' demo could have been a great full game. Then, with a few good demos under your belt, go to a publisher, see what they think. If they like it, your in, if not, 'If and first you don't succeed, try, try, try again!'.

If you do stuff for fun then it is unlikely they will ever find you or bother you. Start selling stuff and it becomes a different matter. Just because you don't make a profit won't make a difference - it will just mean you are a bad business person ;)

Example : gbadev.org 2004 compo (cartridges to be sent soon, 2005). I am pretty sure there are a few Nintendo 'bugs' lying around gbadev, and Nintendo haven't given gbadev a hard time, because they are breaking even on the cartridges, selling them for about USD$14. If thye had sold them for USD$25, I think Nintendo would be on their tale.