That's why I recommended to not even bothering with the polishing step since he is just putting them up for sale. Unless they have some pretty significant scratches all over the whole car, a good cleaner wax with another coat of a nicer wax on top will make it look great without the need to get buffing stuff if you are only planning to use it for the pre-sale prep. If it does have some small localised blemishes you want to fix a hand applied compound will work as a cheap alternative.
I like to use Meguiar's cleaner wax if I'm doing a detail that doesn't involve polishing and then after that cures for a day, put a coat of nice wax on top. Here is a picture of my dad's truck that I did. No polish, just a wash, clay, cleaner wax and then Meg's NXT 2.0 for a final coat. The clay had the biggest affect on it I think. Polishing it is on my list of things to do though, but its a daily driver and work truck so its racking up new scuffs and scratches all the time.
Here is our old Blazer right before we sold it. I did the same, just a good clay bar and only one coat of wax for this one and it looked great for the sale and the new owners were really happy with it.
I don't think a professional detail is worth it for a private sale IMO. For a dealer its to be expected, but for a private sale as long as its clean inside and out to a reasonable level, its good enough.