I'm fairly placid when it comes to selling + buying horses, donkeys, unicorns, pegasus, winged unicorns, and exchanging items.
Here are a couple items I'm currently in search of; Golden Apple - Wandering Spirit - Helios Ray - Fertility Wand - Philosopher's Stone. 
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Here are a couple things about selling horses to me.
 Must be pure-bred / not a mixed breed' horse. 
If it is a draught horse / Donkey, it must be under the age of 2 years 6 months, for both breeding & BLUP purposes.
I will not buy horses over 15 years of age, unless it has the Philosopher's Stone.
When it comes to horses w/ black market items, prices will always vary; there is no way to tell how much I am willing to pay.
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Purchasing Prices. 
Average (any breed) horses - 2,000 equus / 20 passes.
 Donkeys (females) - 50,000 equus / 100 passes.
Donkeys (males) - 40,000 equus / 80 passes.
 Draught horses (females) - 100,000 equus / 80 passes.
Draught horses (males) - 80,000 equus / 75 passes.
 Draught unicorns (females) - 250,000 equus / 150 passes. 
Draught unicorns males) - 200,000 equus / 125 passes.
Unicorns (females) - 150,000 equus / 80 passes. 
Unicorns (males) - 100,000 equus / 75 passes. 
(I may or may not need to adjust these prices)
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I will not be selling any horses from the "Favourites" tab, nor any of the Akhal-Teke breed.