Well, to be fair bikes can be returned and generally have a manufacturer’s warranty. Horses have nothing like this. Also, you don’t have to worry about your bike being lame, taking off with you, having potential gait issues, undisclosed issues on examination, etc…
I don’t have an issue paying some type of commision if the trainer actually does the job and helps close the deal. A trainer can help make decisions for you, like, “Is this bone chip something I should worry about? If not, how much does it impact price? Is a little arthritis in the coffin at age 9 a deal breaker? This lead change isn’t finished - how long will it take to complete it?”
The bone chip likely means the horse won’t be insurable on that leg. You need to consider if you are okay with that.
Arthritis will take maintenance. You can factor that into your price (and $10,000 may save you $$ in the long run depending on the finding).
A lead change? You might be okay with it, your trainer may find the horse is changing incorrectly and needs to be retrained (could take a year to fix that one if you have to fix a front to back to back to front). So, you have to calculate the cost of training into that equation if you can’t fix it yourself.
A trainer can help you factor in the findings and come to a decision. Sure, some may not need that, and it’s not foolproof, but a trainer can at least help ensure you’re getting something close to what you expect and that you’re protecting your investement.
I have never, persoanlly, had such issues with a bike (or a car, or a house). Not even my Orbea Onix (and that was a nice bike!).