This is going to be tough since it is cold outside you will not be able to give her a bath, but there are still ways you can clean her.
Start by currying her and brushing her off the best you can. Soak some towels in hot water (just water, since you won't be able to rinse out the soap), wring out the rags so they are just damp, and then rub her all over with it. Do this inside a barn if you can so that the wind won't cool her too quickly. She won't get very wet from this, but still rub her all over with a dry rag so that she is completely dry. You can also take wet wipes (like what you use for babies) to clean her face and any really stubborn spots, the soap in those wipes are mild enough that they won't irritate her skin if left on. All this may not take all the stains out, but it will clean her up. Make sure her mane and tail is all combed out too.
To top it off, you can rob baby powder in her coat to give it a whitening effect. Plus it leaves her smelling fresh and feeling clean to the touch.
That's the best advice I have to offer. Just make your best effort and I am sure the potential buyer will understand.
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