This is exactly what I do. First thing is to require someone to put something specific in the subject line- that gives an easy filter. Next is to provide a pretty detailed spec so they know that they're not dealing with an ignoramus (either that or a verify that they are dealing with an ignoramus lol).

But I only outsource pieces- plug-innable- of a project. If you make the analogy to OOP- I write the interfaces and outsource the coding.

Often I'll have them modify or incorporate some existing, third-party (like Adam Shaw's calendar- case-in-point).

I've had pretty good luck overall. So I would definitely be willing to use a good site. Hell it might be fun to pick up a couple of gigs there.