I used road planings which, over time, levelled out and worked a treat - it helped that we were a working farm so the heavy machinery helped with this. I laid down some large patio slabs on a bed of levelled sand
around the entrance of the stables and as an area for the farrier.
Don't worry, people know that it's all part and parcel of buying / selling horses. Nothing is ever a done deal until you've actually picked up the horse and if it were me i'd have appreciated your honesty and be happier knowing that you hadn't taken my beloved horse and got into trouble further...