So bassically what you are saying, you sell two sets on a level 30 Arc (he has level 80 Arc) and you sell both each for 400FPs. That makes 800 FPs. Congratulations, the buyer gets in return 798FPs and he paid 2 NET FPs for all the goods you gave him.
If you work the same way in the real world, I am sorry but you would be always losing and losing.
The basic concept for NOT losing a single thing is called NET of something. That is the total when added and substracted from the prize.
In FOE it's called NET FP and also NET Goods. So... I actually don't know if you undestand but you will always be losing and the buyer would always be on + because of your poor concept about how things actually work in the game and real world.
PS: In my world a lot of people search for me because of my fair trades and I am reccomended thru all the people searching for goods... There isn't a single one that hasn't agreed that the PRIZE is something I owe on my GBs. And all of them have agreed to pay it and no one ever disagreed with that.
My point is if you want to sell goods for 2FPs, go ahead, I am not a fan of it.