Auction items should be 75k worth of value which means you would have to trade things to add up to 75k in order to trade for them.
Holiday things, like easter items, should be worth the original value+ 2.5k for every month that passes after the shop sold in is closed. For example, the spd dagger at the moment would be worth 5.5k.
Sellables would be used to buy without trading a regular item and would be worth the original value.
This is so that only fair trades are done as for the fee, I suggest it is counted by time used.