Just a quick thought -
OP, true wisdom is being aware of what you do not know.
When I was (what I’m assuming to be) your age, I wanted to be on my own so bad to prove that I was or could be a legitimate pro.
Looking back, that was the worst decision I could have made. The name of the game in this business is experience and connections and willingness to learn. Skill, talent, money all help, but without those other three, it’s a tricky road.
My advice would be to be honest with yourself about what you could glean from working with the US Trainer. What sort of connections, advice, etc. would you be able to get from her?
If the potential purchaser makes an offer on the horse based on PPE findings, will you counter? What if the horse hasn’t sold in a couple months? Do you reduce the price? Advertise differently? Get it in front of different people? What happens if it gets hurt? When do you cut your losses, when do you draw the line at expensive diagnostics?
If you’re prepared to deal with these situations (+ a thousand more) on your own or through your contact in Europe (be aware that a contact 5000 miles away is helpful, but not the same as eyes on the ground), then separate yourself from the US Trainer and haul in for lessons. Make it clear that you’re stepping out on your own, even as an ammie, so that she doesn’t try to get a slice of the pie.
But what I would do is approach her about some type of mentor relationship. Ask her to teach you the ins and outs of this business with the future goal of you being on your own after X years or X number of flips. Tell her that in exchange you’d be willing to give her a 5% commission and then see how she negotiates.
You will no doubt get a good reputation faster, sell more horses, gain more experience and connections and learn more with her help. Keep you eye on the end goal, not how you feel now, and make the smartest decision on how to get there. It’s not about selling 1 horse, it’s about how to set yourself up with the best opportunity to succeed in the buying/selling business (which is NOT an easy business to break into and is fraught with risk!!!), which means you WILL need connections, experience and a good reputation. Can you build that on your own?
Having a “help” network is not showing weakness, it’s showing strength and wisdom.
PS. ALL good pros exchange ideas and help each other. You want to build a foundation where you’ve set yourself up in the future to call the trainer and say “hey can I get your thoughts on this?” and have her not nickle and dime you because you’ve shown her that you value her opinion and her help and that you’ve worked your way up to be on more equal footing.