I think what you are really advocating is that in a the markets you are active in, you don't want any competitors but you want the advantages of low priced goods and efficient, competitive markets in all the others. Unfortunately every time there is a thread about undercutting that it what it amounts to - it's just not going to happen.

You can also avoid that "barrier" because it doesn't apply to production, only on sales, and it only applies to sales on the market board. I think it would lead to more crappy shouting spam in towns which personally I think is a huge step backwards. Also, if it's a fixed % of an items listing value it's not really a good barrier because there is nothing to stop me listing and selling at 1G to avoid it (although that's likely to be unprofitable, there is nothing to stop me). A true barrier would make it impossible for me to make a sale (ie. you can only use the markets boards having cleared Bahamut turn 5).