Clueless people are not really worth the time to rewrite the exact same thing so here's a quote of myself :
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The weight of the tax will be paid by both the sellers and buyers. What will determine who will pay the most of it is how much people are willing to pay/earn for heirloom points. And that is likely to dwindle or at least increase slower than before with money going out of the system due to this tax.
When put in perspective with... this :
Yeah it's the same principal as when the gubment decided to tax the shit out of tobacco and oil companies. Guess who pays for it? The customer. Cigarettes and gas prices simply go up.
Same thing goes for the Marketplace, it's already happening.
Cigarettes (any drug in fact) and oil customers have a very low demand/price elasticity. That's why the buyer is hurt more than the seller when there's a tax.
Heirlooms, on the other hand, aren't like oil or drugs. You can play cRPG without looms, you can't go to work without oil (and even if you can you can't live without plastic). So it is likely that the heirloom demand is quite elastic, putting the load of the tax on the producer.
Also, the demand of heirlooms is very different compared to that of oil in the way that customers have very different profiles. If you are impatient, you will certainly pay more than the buyer that waits and searches the good deal (just like sellers that is).