Oh yeah I'll definitely be watermarking all WIPs from now on; I was really naïve when I first started taking commissions, and I thought that if I was friendly to commissioners, they would be friendly back. It never occurred to me that I could be seriously taken advantage of; I've had a few somewhat rough experiences, but this one has definitely been the worst.
If any of you guys don't watermark WIPs or don't charge at least partially upfront, you'll be taken advantage of; please learn from my mistakes, guys ^^;