Illnesses that hide: pasteurella, e.cunniculi, Coccidiosis, etc
Illnesses easily missed: vent disease, ear mites, sore hocks, abscesses, etc.
This is why isolation is so important. The stress of moving from one place to another is often just what is needed to bring hidden illnesses to the forefront. As well the time in isolation, when you are watching your new bunny closely, can show you things that you might have missed when you first looked over your new rabbit.
Where to place your isolation cage
At least 20 feet away from other rabbits.
What to do during isolation
Watch for any signs of illness. Snot, hunching up, weepy eyes, unusual fecal output, bloat, etc.
Give your new rabbit a thorough check over once a week. Look for contusions, abscesses, sore feet, overgrown nails, blisters around nose or vent, teeth and more.
What if your new rabbit is sick
Generally after 24 hours the most you can get from a breeder is helpful advice. Suggestions about how to help the rabbit, or more information about condition, and so forth.
If you have managed to "get taken", or involved with an unethical breeder, all you can is cull a serious ill rabbit (pasteurella is one such illness), or treat (most other illnesses). Only you can decide how much effort to put into helping your new purchase.