DO vet and farrier checks
Make sure to get references and make a *no-slaughter/auction* contract.
We have traded for and away many of our school horses. It's can be a very good situation where horses are swapped out of disciplines which they weren't as well suited for and into ones that suite them better.
For example we had a pony who couldn't jump very well/high enough to compete as a medium, and we swapped him to a western family which was much better for him with fewer people and being western. In exchange we received a wonderful (now) hunter mare who was too hot and long strides to be happy western and has a beautiful jump.
Just like any time you are selling a horse make sure you know a bit about the person and that you can trust them.