They don't just use level 1 characters. If you have any experience with RMT's and their advertisement policies, they use stolen accounts which may have characters ranging from 1 to whatever the max level may be in any MMO.

On an average day, I only need to remove about 10 from my list which takes about 15 secs, which is usually more than enough to block the days worth of RMT tells. I hear some servers require more but that is because there are probably a lot more gil buyers on those servers and it would make sense to advertise heavier on those servers.

The only true way to knock out gil sellers is to knock out the consistent buyers.