I'm going to replace the word 'tax' with 'service charge' to hopefully avoid the anathematic perception of the word tax.
First off, you have to understand that the game endlessly generates zulie.
Originally, this endless generation of zulie was kept in check by simple systems like Clan upgrade costs, refine and repair costs etc. That however would only last so long, seeing as it was not expensive enough to take out as much as was being put in.
This service charge however would continue to take zulie out each time it traded hands. Essentially, you'd have to imagine that each time you engaged in trade, an 'invisible NPC' charged you a portion of the profits for using those services.
You could imagine thematically, that you have to pay a certain cost for 'permission' to vend / trade in ROSE to the game itself, because you're bypassing the NPC system that normally keeps zulie in check and removing it from the game current money pool over time.
Like I said, this system would actually impact new players very little. New players typically get gear from drops, NPC's, crafting and quests. They buy food from the NPC's and make zulie from drops by NPCing junk MAT's. They then move onto Dungeons and the like.
None of those processes would be affected by the service charge at all.
In fact, the only way to be charged this hypothetical service charge is to engage in trading items for zulie, selling or buying items in a shop for zulie, or moving zulie between accounts. You wouldn't be charged [or rather shouldn't*] for storing zulie in your personal account storages. As thematically, you're not engaging in the market.
Let's use some number to more appropriately illustrate the point.
1: Newbie is leveling in a various map. They generate 150,000 zulie in zulie drops.
They pick up an inventory full of various blues and materials. They then sell those items to the NPC and make 1,200,000 zulie.
They then purchase some food for 340,000 zulie.
(1,200,000 + 150,000) - 340,000 = 1,010,000 zulie.
(amount deducted from service charges: 0)
2: End-game player is farming Dungeons. They generate 530,000 zulie in zulie drops.
They pick up a various load of chemicals and catalysts. They then vend those materials for 550,000,000 zulie (10% service charge: -55,000,000)
They then purchase food from a vendor for 5,000,000.
(530,000 + 550,000,000) = 550,530,000 zulie gained.
-55,000,000 + -5,000,000 = total profit: 490,530,000 zulie.
[[ Vendor charged service fee: 500,000. Total vendor profit: 4,500,000]]
(amount deducted from service charges: 55,000,000)
3: End-game marketer has a buylist with a total value of 2,950,700,250,000.
Total service cost: 295,070,025,000.
They then sell those items for 4,375,800,190,000.
The players who buy those items from the Vendor, are now charge a service charge, not the vendor, so the vendor now no longer is incurring any costs.
However, I'll still calculate it, to illustrate how much zulie is take out of the market.
Vendor Profit: 1,130,029,915,000.
Service Charge paid by Vendor: 295,070,025,000
Service Charge paid by Player: 437,580,019,000
Total service charges paid to system + subsequently permanently deleted from game economy: 732,650,044,000.
As you can hopefully see. This change would hardly affect newbies at all. The only aspect I didn't cover was newbies buying things like Reinforced sets, which I WILL cover right now, because I know someone will have a complaint if I don't.
Newbies buying Reinforced sets would effectively still cost the same for them. As when they buy a Reinforced set from another player or vendor, the person receiving the money will be deducted the amount, not the purchaser of the item.
The 'concern' comes when you think about how those newbie will be getting the money to buy them in the first place. and there's 2 options.
Option 1: They craft them themselves, either with an Artisan/Dealer they make, or if in the future Professions do come around, they can use that option.
Option 2: They sell items to other players for money, face the minor 10% charge, and then buy them.
Option 2 presents the issue or concern of, will they be able to make enough money to buy them, and the answer is yes. Reinforced sets are cheap, VERY, cheap. And you do not need a unique to level quickly. Even if you did, those are also incredibly cheap.
Over-time however, the service charge system will deduct more and more money from the total in-game amount, making vendors not able to maintain the 'high' prices they know now. Eventually their prices will drop, and the newbie will gain more buying power from typical actions like NPCing blues and materials.
It would actually continue to affect the market until a pivotal point has been reach. That point being when the option of using NPC's to sell items to, instead of vendering them is a competitive method to making money. Once that point has been reached again, then players will have an option to voluntarily participate in vending or using the reliable NPC option. BOTH will continue to recycle the zulie out of the game, thus providing a stabilized market.