Customizing the Market Block currency works very simply.  Each config option is a number that corresponds to a Vanilla Minecraft item/block.

All currencies have the option to be set from 0 to 4 corresponding directly to Emeralds, Diamonds, Gold Ingots, Gold Nuggets, or Iron Ingots.

Seeds also can be set to 5 which is Wheat Seeds.

Temperate Saplings can be set to 5 which is Oak Saplings or 6 which is Apples.

Tropical Saplings can be set to 5 which is Jungle Saplings, 6 which is Apples, or 7 which is Cocoa Beans.

Coniferous Saplings can be set to 5 which is Spruce Saplings.

Each of the animals can be set to 5 which is an Egg.

Example Config:

I:marketcurrencyBonemeal=4
I:marketcurrencyChicken=2
I:marketcurrencyCow=2
I:marketcurrencyHorse=2
I:marketcurrencyPig=2
I:marketcurrencySeeds=5
I:marketcurrencySheep=2
I:marketcurrencyconiferousSaplings=5
I:marketcurrencytemperateSaplings=6
I:marketcurrencytropicalSaplings=7

That config shows that the Market Block will sell the following items for the following currencies:
Bonemeal for Iron Ingots
Chicken, Cow, Horse, Pig, and Sheep Spawn Eggs for Gold Ingots
Seeds for Wheat Seeds
Coniferous Saplings for Spruce Saplings
Temperate Saplings for Apples
Tropical Saplings for Cocoa Beans