Do your homework on the person who will be representing you, are the professional, what can they offer that you cant, etc -- get a contract IN WRITING that you both agree on that spells out all the specific terms that you intend to agree to, ie Health Guarantees, Height Guarantees, Live Foal Guarantees, on & on. Make sure that the contract is specific for EACH horse being represented. Put a beginning and an end date on the contract. Make sure that you address where disputes will be resolved - ie -- YOUR backyard, not the agents or the buyers! Make sure that the agent is taking commission on only the specific listed horses, for "such & such"a time period and that their involvement does not extend into the millineum and cover every horse on your place now and into the future! If the agent is going to actually HAVE the horse in their possession, specifically detail your expectations as far as care, and indicate your approval of use of the animal whether it be showing, breeding, transporting, medicating, etc.
In other words -- there is a lot to take into consideration -- these are just a few things that flew into my head and I didn't really think about a lot more things that may be possible "hiccups" to a situation such as this.
Just my .02 worth
Stach