The problem with most writing jobs on the Internet is they are often done on the fly. You reply to a Craigslist ad or something from a writing website and soon you wash in a flurry of e-mails and fast deadlines.
If you are lucky, then you can develop long-standing relationships with people who will come to you again and again. I recently had a person that I've worked with many, many times before contact me. I had never had any issues about completing the project before getting paid.
It's a courtesy I do for some of my clients. We always hammered out the details about when and how much before I started and it never was an issue. Recently, I had completed a project and she sent over a payment significantly less than what we agreed upon.
She said that the person that paid her only gave her that much and that was all she could give me. Needless to say, this didn't sit well. We writers depend on that money because it's part of our income and when it doesn't all come in, then we get put into a bind.
I told her that I needed that money and that if she needed a few more days to get that would be fine. Days turned to weeks, and soon she wasn't even returning my e-mails. It's obvious that she never got the money and didn't want to tell me. About a year goes by and she e-mails me again about a job. I tell her that given the last transaction, I didn't trust her and that I would need the payment up front. That's how we do it from here on out.