My trainer had a horse that would *not* back out of a trailer. Just plain would NOT, no matter what or who tried. She spent nearly 30 hours in a 2 horse, straight load trailer before she finally came out flailing so hard she fell and slid under the trailer slicing herself up.
So she bought a slant load trailer, just so she could turn this horse around and get her out safely. When she sold the horse, it was under the condition that the buyer only use a slant load trailer. Buyer called her two weeks later, he'd stuck her in a 2 horse straight load and she'd been in there for 48 hours and still wasn't coming out.
If the seller tells you they won't back out, I wouldn't be buying a trailer you cannot turn them around in. You might be able to fix the backing out issue but what are you going to do if you can't and you now have a horse that's stuck in a trailer?