Engaging a freelance designer online can be a bit scary: someone in a different time zone, who you cannot meet in person... Here are some thoughts (in random order) that could be useful both for freelance designers, and their (potential) clients.

• Designers prefer a smaller group of clients who come back regularly over 1-off jobs. There is a lot of switching / friction cost involved when taking on a new client. So you are unlikely to find designers who will deliver an incredible sales pitch, do a crappy piece of work, then over charge and hope to get away with it. 
• Good designers are busy, so if someone cannot meet tomorrow's deadline, it might be a good sign.
• It is too time consuming and stressful for designers to stage a huge price negotiation dance for every project they do, so most will quote their rates, without padding in anticipation of haggling
• if the work load is highly unpredictable (you have never worked with this client, the scope of the project is not clear), then it is not helpful to insist on a fixed project price. A good designer will either refuse to do it, or add an extraordinary "unforeseen" charge in the budget.
• Squeezing out free work for a freelancer to prove herself probably upsets the designer, better is to risk a bit of money, and do a small test project. A few slides will  reveal instantly what the skills of the designer are. This is also an insurance policy against demo slides that might not have been developed by the designer in question. Also compare this small investment to what it would take to hire a permanent designer, and the risk you run when making a hiring mistake.
• When doing your test project, do not only watch out for the quality of the work, or how quick the turnaround time is. What is more important is that the material gets returned at the agreed time, that the dialogue is responsive. You want to test the way to work together, in preparation for very critical deadlines where you have to rely on this stranger.
• Finding a freelancer online is the first step in a longer cooperation, so the way she responds to feedback and learns is almost as important as the quality of the first work that she presented to you