Part 1:
Trading fees. How about for a certain percent of the melees worth you have to pay. (e.g 15% of the total values of the trade. 50k + 50k is 100k. 15k fee.) or you have to pay $3.99 a month to trade with no or a reduced trading fee. And also add a $0.99 fee per every trade.
Part 2: Or only trade things with no set value, for every damage, every effect and unique purpose to up the new value. Also the trades have to about the same values. (e.g one crab dagger plus a spd dagger = one shamrock dagger and 100 burgers. Values [only examples]
Crab dagger: 10k
Spd dagger :20-40k
Shamrock dagger:30k
Candy cane: 10k
Pitchfork:7k
Burgers:10
A crab+candy cane+1000 burgers=shamrock dagger.