Hi Jessica,
ANY fees on any platform make sellers rethink their selling strategies. Between Ebay, Paypal (Ebay), Australia Post and Ebay's Fees on postage charges there are some large fees being taken from sales already. Adding another tax is just going to make it harder on the sellers. The trading card market prices are primarily dictated by the consumers. Customers will pay what they want for the product in question. Prices fluctuate as the consumer wants the best value for money while the seller tries to maximize his profits.
I think sites like this stand to benefit from this new tax. Whilst the exposure is minimal compared to the ebay platform, its a target audience.
As for changing the prices of cards? Well that's always a possibility. Time will tell on that issue.
That's my two cents worth!
Mark.