Guild Wars 2

If i were to buy an item for 11c and then process to immediately sell it for 13c. do i make a profit of 1c?

this is assume the actual fee’s on the market place is 15% and it rounds down. if it rounds up i would break even.

but i am not sure of how the system rounds, and there lies my question.

No, because there are 2 fees that are applied separately, and the rounding will not work in your favor.

List item @ 13c -> Listing fee 5% = 1c (rounded up to the minimum amount)
Item sold @ 13c -> Sales tax 10% = 1c (rounded down from 1.3c)

So you would receive 12c back, but have paid 11c for the item and 1c for the listing fee, effective breaking even.

Edit: see also http://wiki.guildwars2.com/wiki/Trading_Post#Fees