The other answer is to help thin down the ammount of venders. People will not vend as much when there is a tax involved, when you receive even less of a profit margin on re-selling that NPC Tool Shop red potion you're more likely to not try to do it. There are a few people and store chains that have fought against the small profit margin and are able to make money, but for the most part it's a way to make sure that there are less venders that mindlessly vend everything in their storage wither there is a market for it or not. Also, it's an encouraging factor for players to use the forums to sell and trade products because there is no tax on using the forums and being active here. The logic is that if you're going to sell an item for 100mil you're going to want all 100mil and not pay the tax as part of it.
The RP answer is- it's a tax to clean up after all of those venders and make sure the streets of Prontera (and other towns) are well organized and clean because you guys are so messy.
Tax rate 
I'm displaying my noob side on the forums again. 
