When buying a horse, how do you ensure you don't get ripped off and lumbered with a poorly horse?

unfortunately you can't, I had my horse had a 5 stage vetting and also had 2 other visits from the vet as it was though he had ringworm (he did't in the end just a random bald circle on his nose). However after I finally got him onto my yard I found out he had mites and really really bad mud fever on his legs, however 3 different vets didn't spot this.