Whatever the arrangement, think no way it’s going to work unless trainer is on board with OP running sales horses thru his/her business. This may or may not involve commission but needs to be brought out in the light BEFORE this business plan is finalized. It is trainers facility and this won’t be an occasional owner selling a horse privately.
Perhaps the title of this thread should be " what boundaries might my trainer put on me running a sales business out of their barn?" Particularly when they also buy and sell as a big part of their income.
Its possible to make this work if you import the right horses at the right point in their training and present them to a substantial buyer pool ( like WEF) and can sell them quickly. BUT…I don’t see how it will work one horse at a time while boarding/training at full price in an active show barn and I am assuming going to shows under that barns care.
If OP owned a small farm with decent indoor and a full set of show jumps bringing in a private trainer and brought several over at a time? Think that works much better then full board and training sucking at least…what…? Couple grand a month? 24k a year out of the potential " profit". That on top of vet, farrier and show expenses and on the road trainer fees.
Word to OP…there are numerous " wink wink cough cough" ownership schemes out there, don’t be surprised if some raise an eyebrow if you share you have an " arrangement" that allows a sub market purchase price and you split any profit with them…assuming there is a profit which is a major assumption.