Request paid help on a medium size project that mainly involve creating something new (moderate in size) or making time consuming changes. Examples include creating a new script, adding new features to a script, creating a small web site, installing a CMS etc. The task should not take longer than a few days maximum to complete- otherwise post the project in the "Large Projects" forum instead.
Request bids to work on a large project in terms of complexity or functionality, such as creating a web application, a site clone, or long term sustained work on a task or issue.
Request bids to work on a project where you're not sure how much it should cost to properly implement. In other words, bidders will offer their own quote in their responses.