There is a limited supply of everything, although you can order more items than I have unless the order is too big.
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Dragon Head (20 available) 1 db each
Elytra (0 available) 2 db each
Shulker Shell (33 available) 2 diamonds each or 4 for a box
Saddle (4 available) 2 diamonds each
Podzol stack (2 available) 5 diamonds for a stack
Cobweb stack (1 available) 5 diamonds for a stack
Emerald ore (16 available) 2 diamonds each
Bat spawn egg (1 available) 5 diamond blocks
Eggs (18 stacks of 16 available) 1 diamond for a stack
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Random piece of diamond armor with random enchantments: 3 diamonds each
Diamond pickaxe with random enchantments: 2 diamonds each
Diamond sword with random enchantments: 2 diamonds each
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Gold, Iron, Emerald and Diamond Trading
5 gold=1 diamond=13 iron=2 emeralds
You can trade for any of these, although some offers may not apply.

Added extremely cheap diamond armor, diamond swords and diamond pickaxes with random enchantments and updated the availability of items.
Also added gold, diamond, emerald and iron trading.