We had nightmare buyers for our last house and I am not at all ashamed to say that we didn't clean anything before we left. It all started off so well - they offered the asking price so we took it off the market straight away, then there was delay after delay after delay.
They threatened to pull out of the sale unlesss we knocked £20k off the price once they learned that we had put in an offer on another house (and obviously didn't want to lose it). Then we had some works done on our roof which we told them about and they insisted on us paying for an expert of their choosing to survey the works. We refused to pay their costs on the basis that they obviously decided not to include the roof in the survey they paid for which came out fine. This all went on for months during which I had a miscarriage which I blame on the stress of the whole thing.
They insisted on second vists on 3 occasions which they then didn't turn up to. They used to stand outside and stare in the windows while we were at home. Then on the day before we were due to complete, they insisted on a 'pre completion inspection' or said they wouldn't complete the following day. We had the movers in, the house was in a complete state and we were stressed and knackered but had to go down to the pub while they did the inspection. Only the wife turned up and was there all of 3 minutes apparently so it was obviously just another inconvenience to place on us.
I warned the EA that we weren't going to clean the place and he said he absolutely didn't blame us as they were nightmares for him to deal with as well and would often drop off the radar for weeks so no one knew what was going on.
I still get all riled up when I think about it and love to think about the look on her face when she walked in to the house and it was all messy (note, it wasn't messy when we lived there but there were dustbunnies behind furniture and grimy marks everywhere where the movers were trudging in and out etc).
So the only thing we left them (besides the instruction manuals for the boiler etc) was a note saying 'We would have taken £15,000 less'. 