from:
http://wiki.guildwars2.com/wiki/Trading_PostPurchases on the trading post must be made using coin and there are two fees applied to sales: 5% of the asking price to list and a 10% tax on purchase.
Fee notes
The listing fee is shown before you press the submit button, but the sales tax is not shown (neither in the sales window nor in the pickup window).
Both fees are mathematically rounded, i.e. fractions less than one-half a coin are rounded down; fractions above that are rounded up.
Rounding examples
Selling one item for 29 Copper coin should generate a 1.45 Copper coin listing fee; you pay 1 Copper coin instead.
Selling another item for 31 Copper coin costs 2 Copper coin, i.e. rounded up from 1.55 Copper coin.
Losing potential earnings
Be careful not to list for so little that you would take home less than you would get by selling the item to a vendor, i.e. never sell for less than ~20% more than the item's value. For example:
An item of 88 Copper coin can be sold on the trading post for 89 Copper coin, but this results in 76 Copper coin (after deducting 9 Copper coin for a successful sale and 4 Copper coin for the listing fee).
If offered for 1 Silver coin 26 Copper coin, you would take home 1 Silver coin 7 Copper coin, 19 Copper coin more than the price the vendor offers (after paying a sales tax of 13 Copper coin, listing fee of 6 Copper coin)