Logo design is already one of the major categories on freelance work exchange Freelancer.com. If you don't just want to post a design specification and sift through bids, a new "contest" option lets you invite submissions, and you pay a contest fee to the winning design.
The process (not entirely dissimilar to existing models such as 99Designs) works like this: you lodge the specification and pay a contest fee (either $290, $490 or $690; the more you pay, the more widely promoted the contest is). The site then promotes your pitch to freelance designers on the site, who submit a completed design. You choose the one you like best, and pay the winning fee. If you like more than one design, you pay the winning fee to each relevant designer. You can also pay extra fees for faster turnaround, to block your contest from search engines, or to seal entries so other contestants don't see them.
If you get a good design for $290, this seems a reasonable deal — however, you won't have the option of getting the design tweaked as you would with a more conventional brief. If you're a designer, would you be tempted to enter this kind of contest knowing your work might not be selected?