I remember a while ago when we used a plugin that made the players shape the economy and prices.. It was really bad because if you were to sell a lot of stuff, for example lets say diamond. If I was to sell it to the shop for 200nbz, the price of diamond would go down, meaning the more I sold to the shop, the lower the price got. If you were to sell a stack all at once then they'd all go for the silly price of what 64 single diamonds would've lowered the price down to.