A couple of things that a few here have already mentioned, and a couple that haven't. First off, I've been in this situation 3 or 4 times in the 6 years I've been in business. I can tell you from experience, I won't be reverting a computer back to the way it was and hand delivering it to a customer. My time is far too valuable. We have a $35 diagnostics (which includes a 21-point inspection) at the shop which is waived if they decide to have us fix the computer. I have now and again, reassembled computers back the exact way they came in, and charged the customer $35 diagnostic for our time determining what the problem was.
The first non-paying customer was a good client of mine, was doing some web design for him, and all of a sudden he stopped paying. Claimed he was having a rough time with the starting the new business. He owed reoccurring payments monthly. After two missed payments, I shut off the site I was designing.
Second time, was a few months later--same client. He brought in his laptop for repair. Needed a new HDD. I finished repairs, and just like the website, it just sat there and sat there. 2 months went by, without a word from him. So I left him a warning voicemail that I was going to sell it. And then sent a certified letter requiring a signature. He refused to sign for the post lady so it was sent back to me after 3 attempts on their part. So it was sold. 3 months later he asked if I still had it. I politely explained to him that, no, I put in a new hard drive, and performed data recovery on the failing one. It was a bit of work and sold it to recoup my investments in time and parts. I'm not a dick to clients, but I try to be firm about getting crap picked up. I don't have tons of storage space. I struggle with enough space in our 900sqft office everyday.
The last time was a year ago, had a client bring in two laptops with all the keys ripped off by one of her kids. She was difficult to get a hold of, but didn't think anything of it as she seemed nice and respectful. Well we completed the repairs, the bill came out to exactly what we quoted her. She said hold on to them, I just had an unexpected bill come up that I need to address. Give me a week. A week later, called and called. No answer. Finally a month later she answers all upset because she claimed we took too long to do the repairs and the bill is much higher than when we first talked. It wasn't, she was just buying more time. 2 weeks later and several voicemail and her fiancé answers the phone. Gave me another sob story. Buttered me up and said they will pay it all as he knew we were being honest and patient. Asked to give him 2 weeks. Two weeks go by, he calls, sets an appointment, never shows. 3 weeks go by, I was about to sell one of them, had a buyer lined up, and he just said, forget it, he can't afford it instead bought her a new computer. I thought, "What??? Fixed vs new?? Clearly the new is $400, their repair bill is 387 for two working laptops." He asked if I can at least get the documents off for him and we can just keep the two laptops. I agreed.