From what I have learned the
best stall leavings are horse manure and straw and almost nobody beds down with straw anymore, according to Colonial Williamsburg. Since I am studying composting, NEXT winter my horse's shelter will be bedded down with straw.
Most horses are stalled today. It keep their hooves healthier than
turnout with ONLY a shelter. Been THERE, done THAT, and HATED IT! 
After I got my horses to my own place with them in the back yard I made stalls and I went to sawdust and/or pine shavings and Equine Fresh which is also 100% pine, dried out in pelleted form. That keeps the ammonia down and is healthier for their lungs. Straw is still readily available and my hay man sells them for $3.00/bale.
I try to put up about 50 bales every year and it is a nice winter topping for the spots where my horses choose to poo in their stalls. Also my mare's stall can have a light draft from the door to the shelter, so she likes 3-4 flakes of straw on the coldest nights.
If you do not turn or till stall leavings with pine shavings they don't break down quickly and are better used as mulch. I have even dug into a pile that was 5 years old and found intact medium shavings.
You really SHOULD ask about two things before you take away their stall waste:
(1) What is the bedding being used
and'
(2) What the horse is being fed.
I suggest that you look for a high class show stable or one where people board and pay a trainer. They will usually shell out big bucks for expensive feed and now mostly feed pelleted hay.
NO Seeds.
Almost ALL of us now feed pelleted grain.
NO Seeds.
Also, you can work on ANY stall leavings by turning them:
http://www.rodalesorganiclife.com/garden/how-compost
I COUNT on my stall leavings to sprout pasture grass, so I feed straight hay.
If I thought there was a good market I would try to create seed free aged compost and sell it.
There is ALWAYS somebody selling Preen............