Didn't the buyer have an inspector? If so, let him disclose his findings and then figure out your next step. I was always told that most houses face natural settlement here and there which produces fine hairline cracks. I still think I would leave it up to the buyer's inspector - after all, it is visible right?

Good luck

The inspection has already been done. However, these cracks are still developing and I have no idea if their home inspector saw them - I didn't even notice them before! I'm sure they're worse now and who knows how much worse this is going to get before closing.
I just can't figure out why this is happening now, all at once.

If the house were on a foundation, I'd start watering the perimeter. Foundation company once told me that everyone gets natural settlement and with unevenly watered grounds, it can increase settlement sooner than later - thus cracks in sheetrock, etc. This guy told me to remove the gutters to allow rain water to flow evenly off my roof and/or to keep the perimeter watered 2 to 3 foot from the edge of the foundation.

LOL, maybe you could start watering more and swell up those cracks? I just don't know. Foundation problems scare me to death. good luck :)

Interesting - I doubt very much the ceiling's drywall because it takes a lot to make that cardboard split - no doubt it's fibrous plaster or cast plaster.

Irrespective of what's causing it, (don't overwater the foundations either) make sure it's not too dry indoors, humidify a little if it's well under say 45%

The cracks themselves can be sealed with a FLEXIBLE gap filler, make sure it's a flexible water clean up one, force it in gently with a thin flexible scraper, and you can gently wipe over with a slightly damp cloth to remove any excess (if you work carefully with the scraper there won't be much excess) and see how you go. if it's a white ceiling, you may not need to repaint.

There is a boat-building technique to stop cracks, basically you drill a hole in the path of the crack - being round, when the crack reaches the hole, it'll stop.

I'd exercise caution when applying this technique, though, not knowing what the ceiling's made of, condition etc. If you drill small holes, say 1/8 inch, (you will need to be accurate with position) you should be safe enough.

Do you have access to the ceiling space above? In older places with lathe and plaster and plasterboard, they often sag with time, in extreme situations, you can prop up the sag with a broad piece of plywood, supported from underneath, then up in the ceiling place (after removing insulation and dust etc) pour liquid casting plaster - plaster of paris - over the top of the sagging part, and allow to set (don't pour it into any light fittings, vents or electrics) - this will reinforce/hold up the ceiling, I suppose if you were clever you could reinforce the plaster with some sort of material like you were fibreglassing, even with, say, fibreglass cloth? (you want to avoid flammable materials up there)

Then, the idea is, you have a solid, no longer sagging ceiling, and you apply a thin skimcoat over the underside, to seal/cover the cracks.

Just had a 2000 ft modular put up in November 07 where I did not get my CO til the end of February 08. Lots of major issues. Never modular again, ever. Yesterday I found two cracks across my dining room from wall to wall. The adjoining porch has a dip from what I believe are trusses or beams put the wrong way due to sloppy workmanship. The porch dip is out and 10 feet parallel to the side. My question is my husband (175lbs) who had been working out on the dance dance revolution mat on the carpeted bedroom floor above. Any relation to this? Have not seen problems concerning the pad being used on second floors on the web. Otherwise I am assuming it all has to do with the inferior workmanship seen all thru the modular.

My cracks in ceiling are many, and diverse. Closet tops have separated from ceiling, kitchen cupboards' doors are getting more difficult to close, and are looking more crookedly hung - when a few years ago, they hung very nicely. Also, wooden floor planks are becoming wider and are all over house. I have a crawl space - how can I tell under the crawl space what kind of problems I might be having, and what to do about it?

Could they have appeared when the inspector was in the attic or inspecting above the kitchen? We moved into a home and had to have the cable company out to run cable lines upstairs via our attic. There was one nail pop we filled in the hall ceiling up there before they came. After the cable company left there were a bunch...not really their fault...