There’s really only one way to be sure you are doing the right thing and know what to expect, ask your trainer.
I’m presuming that you have given it a good thinkover and are onboard with sending your horse up north and have all the pros and cons there sorted out.
Have a sit down with your trainer and tell her what you told us. Say you are not sure how it’s done and what to expect and that you want to make sure everybody involved are happy. Your trainer might say, yes I expect a comission or she might say not to worry about it.
If you are to “take care of” (pay a commission) to your trainer you might want to tell the trainer up north that you want to make xx on the horse and that includes your trainer but she will have to make her commission on top or from the buyer.
It’s also a good idea to discuss, if you haven’t already, what your expectations are with the trainer up north. Tell her everything about your horse and ask her -I’m thinking that I would like to get xx, is this realistic, do you think you can get this much? How long do you expect before you can sell my horse? What will we do if you can’t sell him in this timeframe? Would you be open to a lease in case the horse doesn’t sell etc.
You could also tell her that you would like xx, and you are happy paying the 10% on that number. If she can get more you will give her a bigger % on the exceeds to encourage her to get top dollar for your horse rather then going for the Quick sale if this is more important to you. Also discuss what happens if you end up selling for less then what you had planned.
Should your trainer say that she does not expect a commission I still think that you should get her a little something when the horse is sold since it is her connection and she might be calling in a favor from a friend.
If you end up being more then happy with the sale and making a lot of money, you can again offer to cut her in, or depending on the value of the horse a nice gift could be anything from a good bottle of wine or a dinner out to a cooler in her barn colors or something else she might enjoy.