>>If you lent me some money, and I lent you a guitar, wouldn't you feel it reasonable to hold on to the guitar until I gave you your money back?<<

I once did some work for an IT training company as a freelance trainer.
From my memory, it was about £1000. However, it was obvious that I was going to have great difficulties getting paid. So 3 months later, I went to their offices to talk to the manager. He wasn't in. However, there was a top of the range monitor which was very expensive. So I opened the boot of my car, loaded the Monitor in and drove off.
I rang him the next day and told him what I had done. He went bonkers, but I told him that if he delivered the money to me in cash, I would let him have his monitor back.
That afternoon, he came round with the cash and I gave him the monitor back. Needless to say I didn't do anymore work for that company.