Here is how it would imbalance the economy:
In one scenario, we could assume that the person has no previously obtained kits. They use warrior and eventually get a low costing kit, on of the 300-500 coin ones. Now that kit is a bit more op, so they can get kills easier. Then they sell that kit to upgrade to an even more op kit such as Black Mamba. Then it would be very easy to get black mamba. Well if everyone had a super op kit, then there are many more coins being put into the banks of the players. Since all the players would have generally the same increase, prices would increase by the same amount. Then people complain that it is too expensive, etc.